It feels like they could all be part of one unified platform.

They are. It's called the fediverse.

There's no reason why any of these software options couldn't support all the same stuff, as you say. But so far they have chosen not to.

Maybe another option will come along one day that supports more of it at once.

Facebook. You are describing facebook.

Not everyone wants to use or host everything.

Why are there so many separate platforms in the Fediverse like Mastodon, Lemmy, PixelFed, and PeerTube? It feels like they could all be part of one unified platform.

Simple answer: Because people have different visions, different priorities. Expertise in different programming languages and tooling.

Why do we have three Reddit alternatives in Lemmy, Piefed, and Mbin? Why don't all their devs work on the same project?

1. The Lemmy devs are highly controversial. The other projects don't have such issues.
2. Lemmy is written in Rust, Mbin in PHP, and Piefed in Python.
3. The scope is different between them. Lemmy only cares about communities. Mbin targets wider compatibility with the microblogging side of the fediverse. Piefed plans to one day add microblog support iirc, but their priorities lie elsewhere.

Some platforms care about interoperability more than others, trying to push for FEPs (basically standardization proposals for the fediverse), while others don't. Some care about privacy even if it degrades interoperability, some believe the latter outweighs the former. Some disagree on how to implement a specific feature.

Mbin adopted Reddit's karma system, Lemmy didn't. Sure you could combine both of those and give the user the choice, but this reflects a difference in design philosophies. Lemmy users don't just lack a karma system, they outright don't want one. It's a system which promotes karma farming, so it's associated with the worst of Reddit. But ironically, it also encourages contributing, which is probably why kbin (Mbin's predecessor) originally added it. The fediverse is in need of contributors over lurkers, so whether a karma system is bad or good for it depends on your perspective. And that perspective differs between the developers of these two projects.

Ultimately, sometimes projects are just born out of a dev wanting to challenge themselves by recreating something themselves. Iirc that's how Minecraft was born, with its creator originally wanting to test his skills at an Infiniminer clone and that spiralled into the most successful game ever.
So why a separate project is started isn't always logical even. Sometimes the dev just felt like it.

I for one like Mbin but dislike Piefed and Lemmy both. But most people seem to think differently, as Mbin is the least popular of the three. There's a lot who have sworn off Lemmy in favor of Piefed, but there's also a lot of people who prefer sticking with Lemmy. If there was just a single option, there's a possibility I or others might not be here today, because we don't like the choices that single option went with.

Finally, there's also the danger of a company acquiring the project and enshittifying it. They can't really acquire an entire federation protocol and every software implementing it.

In the first place, the fediverse is about interoperability between different social networks. If you have just one social network, you have no use for the fediverse anymore. So your question is really more like "why do we need the fediverse?". There's no such thing as "unifying the fediverse", as that's the antithesis of the fediverse. Unifying it would undo it. The fediverse is nothing without its nature of connecting different projects together.

Well, diversity is the central idea behind the entire Fediverse... We get many different perspectives on the same content. That includes many individual instances and individual software. The opposite of that would be no diversity. One platform and one software, like Reddit or Facebook or most big commercial services. And we have projects in between, both federated and non-federated, even crypto-based, which combine many aspects into one platform.

Each platform have different functions and is a different tool.

How do you imagine all the functions of PeerTube should fit inside Lemmy?

All the platforms are already unified. You can subscribe to a PeerTube channel from Mastodon. You can post to Lemmy communities from Mastodon. People on Pixelfed can follow you on Mastodon.

You're pretty much describing Friendica. It does everything you've listed there (and more) except for Matrix.

Mbin supports both Lemmy and Mastodon. Though I have no interest in Mastodon's "microblogging"/Twitter-like format so I've never investigated that, despite being on an mbin instance myself.

Fediverse... Fed... Federated. Unifying it would defeat the purpose. Yes, there could be a single platform, with federated hosting, but multiple platforms working with a single protocol is a good thing.

Consider the web - in the old days, it was an open platform. Then Internet Explorer got a stranglehold, and to use the web practically required using IE on Windows (many sites did not work in other browsers). Eventually we righted the ship, but now Chromium browsers are taking over, and we're heading in a similar direction.

For the fediverse to remain open and effective, we should embrace extra platforms*. It prevents anyone getting too much control over the protocol, prevents lock-in, prevents centralization, etc.

*We should generally encourage use/development of the same protocol, though.

I remember using Trillian to unify my ICQ, IRC and MSN Messenger chats into one application back in the days and once the fediverse matures I wouldn't be surprised if an alternate front end that let's you login at multiple fediverse services and interact with them all appears.
I mean it seems to be the default way things go - "GOG, Epic, Prime? No no, Heroic!", "You're not booking your hotel room at the hotels website are you? You get a better price by using instead." and so on.

Nope hell no don't put all your eggs in one basket, although this would be nice in a client there is fedilab that I think does this.

I haven’t spent enough effort understanding ActivityPub to come up with something coherent. But i often find myself with similar thoughts, although for me its usually focused on “would it be possible to create some kind of service-agnostic ActivityPub client?”

I am for it but only if media can be separated into different tab