Explain Like I'm 5 (ELI5)

There are two levels at which economic growth can (and must) manifest. At the business, i.e. micro, level and at the societal, i.e. macro, level.

At the micro level, Marx already described the rationale guiding the individual Captialist's motivation to seek out growth: stagnation leads to losing out over the competition. If your competition manages to produce more X for the same price as you, they can sell it for less, resulting in your loss of market share and profits. Hence, you yourself are required to seek out technological innovations i.e. boosters of production quality, speed and efficiency, in order to outperform, or at least survive alongside, the competitors. The inherent logic of Capitalism itself nudges you this way rather than individual vices.

At the macro level, we are not looking at individual firms anymore, but at the financial/economic makeup of a capitalist country. Here, there are a lot of structures in place which necessitate a perpetually growing economy. For example, as per Keynes theory, a state of full employment can only be brought about if investment (i.e. the spending of money in return for more money in the future) takes places at a steady rate. That is because employers only hire new workers if they expect demand for the product they sell to rise in the near future (relating the full argument here would be too complex). If investment were to dry up, the economy would slowly grind to a halt and fall over like a bike, which in order to stay upright, requires forward movement.

You are not wrong if you consider companies still owned by people who work inside the company and thereby have a certain connection to it.

The problem gets introduced at latest by shareholders and investors. They have no connection to the work of the company and literall just paid a certain amount of money to own a part of the company. Now why would they do that? To earn more money. And how do they decide whom to give their money to? To those who show the most promise of more money. (You could say its the product of possibility of payout times the amount of payout. Higher payout makes people more likely to take higher risks. Greed destroys reason after all). So those companies that promise to cut spending by 10 % and give more money to the investors are thereby more interesting to investors than company B that just says: we will not lay off people. And you nearly always need investors in order to tackle bigger projects.

How do you start a company? For most people who don't have the funds themselves (and if you do, you very likely already profited yourself from the parasitic side of capitalism) You get somebody to invest in your idea. And those investors want to have something in return for their money of course. Some countries have governmental investment funds in order to alleviate the parasitic influence of the shareholders a bit, but its not enough.

That completely disregards the whole political side of capitalism. Money is power and a few people with more money can thereby influence the political landscape in their favour to get even more money.

So in basic principle: its greed, simple as it sounds. Rich people are NEVER satisfied with their amount of money. It seems to be pathological, why else would you even reach a billion Dollars? Normal people would peace out and enjoy life.

Lots of people look at our current economic system and call it and everything with it capitalism. But it's not really. Nothing in capitalism requires infinite growth. That's just a byproduct of the public stock market. Public shareholders buy shares in the hope they will be worth more in the future. If the shares of one company aren't, people won't buy it. And since the assigned value of that company is tied to its share price (kind of a silly idea if one really thinks about it) that company will have an apparent value of nothing.

So in order to avoid that zero dollar share value, a company must appear to be growing.

But nothing in capitalism requires a public stock market to begin with.

It's because of competition. Someone else will find a way to produce your product faster, better or more of it at the same costs, with less people, using new technology which requires a higher turnover but has fewer costs, and you will lack capital to invest in that machine to make your business competitive again. Even if you do have it, there are now two companies with that machine who invested capital and require a higher turnover, but the market is limited and one of you, if not both, will fail to bring in the return on the investment, let alone make enough money to invest in the next machine. That effect can be seen all the time, the current best example in Europe is the automotive industry, the next one will be giga factories, AI data centers.

There are plenty of companies that tick along fine at around the same levels each year

Problem is shareholders who contribute nothing except money but expect growth

Because, according to capitalism, growth benefits everyone. Both the wealthy and the poor. The rich get richer, the poor get jobs. They can all be taxed and that tax can then be spent on improving public services and infrastructure, which then also benefits the unemployed. The larger your business grows, the more wealth it generates, the more jobs in provides, the more tax it pays. So, by that logic, growth is seen as the best indicator of success.

No reason, plenty of private companies do that, but they usually stay on the smaller side, so you don't hear about it. Valve (Steam) is a prominent example. If you have the chance to work for one, take it, good chance you'll be treated well. It's when companies are publicly traded and sociopathic CEOs take the reins saying 'I have an obligation to my shareholders' and 'line must go up' that things truly go to shit. So, some combination of putting psychos in charge, the greed of capital and a lack of regulation IMO.