Because it’s an internet enabled device running a fully fledged operating system that will have bugs and vulnerabilities that need to be fixed all the time.
It would have been far better to just have serviceable dumb TVs with small smart sticks, but I suppose that doesn't extract enough money for these poor destitute CEOs.
Because most media is still 1080p, 4k TVs (especially big screens) need to basically have a mini PC inside to handle the upscaling, or you might as well stay with a 1080p TV. Something very few people seem willing to do these days
At that point the "smart features" don't need anything else, so it wouldn't make sense to leave them out.
When media is mostly 4k, we won't need upscaling and "dumb TV's" will make a comeback like back in the day when people would buy cheap TVs that were just as good as the expensive ones.
Since most modern media is streaming tho. And streaming companies keep charging extra for 4k streaming, it'll be a while.
A dumb tv with a user accessable slot for a raspberry pi compute module would be nice. You could upgrade the brains easy enough and recycle the module into something else.
If you're one of those customers who has to do this, connect it via ethernet, do the setup, then forever leave it disconnected. Never give it your wireless credentials. I personally wouldn't trust the TV to forget the credentials and not phone home and make itself the ad machine the company wants it to be