The fact that phones haven't been able to to this easily/natively/what have you is wild. Similarly, the fact that you can't use old tablets as external monitors without, in my experience of quite a few, significantly buggy software that's got significant lag in the best of times, is pretty wild. Sure, the technical hurdles aren't small, but damn.
I've got a reasonably high end newish tablet (Galaxy S7+) that I can use miracast to use wirelessly as an extra PC monitor. It works quite well... if I'm near to a high quality new router. But can I just plug the tablet in and use it as a monitor with my laptop? Not remotely well.
It's been a year or two, maybe I should check for new software again.