Also to add to this, the life-cycle of a TV display is mismatched from the live-cycle of media playing hardware or just hardware for general computing: one needs to update the latter more often in order to keep up with things like new video codecs (as for performance those things are actually implemented in hardware) as well as more in general to be capable of running newer software with decent performance.

I've actually had a separate media box for my TV for over a decade and in my experience you go through 3 or 4 media boxes for every time you change TVs, partly because of new video codes coming out and partly because the computing hardware for those things is usually on the low-end so newer software won't run as well. In fact I eventually settled down on having a generic Mini-PC with Linux and Kodi as my media box (which is pretty much the same to use in your living room as a dedicated media box since you can get a wireless remote for it, so no need for a keyboard or mouse to use it as media player) and it doubles down as a server on the background (remotely managed via ssh), something which wouldn't at all be possible with computing hardware integrated in the TV.

In summary, having the computing stuff separate from the TV is cheaper and less frustrating (you don't need to endure slow software after a few years because the hardware is part of an expensive TV that you don't want to throw out), as well as giving you far more options to do whatever you want (lets just say that if your network connected media box is enshittified, it's pretty cheap to replace it or even go the way I went and replace it with a system you fully control)