In the life of my last TV (10+ yrs), I've had to switch out that separate box three times. It would have sucked & been way more expensive to have had to replace the TV each time.

Firmware can be updated, sure, but there's the risk of some internal component failing. There's the risk of the services I want to use not being compatible. I'd also prefer to use an operating system I'm familiar with, because, well, I'm familiar with it, rather than some custom firmware from a TV company whose goal is to sell your data, not make a good user experience...

Of course, this ties back to the enshittification of the Internet. Every company is trying to be a data broker now though, because they see it as free passive income.