Because burn in is a terrible thing for a computer screen.
Phones aren't used the same way that desktops/laptops and tablets are. There's no expectation that the user will be interacting with static elements on the screen for hours at a time. Typically the longest on-screen activity you might do with a phone is watch a video or play a game. Both of those match the use case for TVs.
I strongly disagree. A tablet is much more similar in use to a phone than to a desktop/laptop. There is nothing static in a tablet interface or its use that would warrant such a concern. The tablet also doesn't stay plugged in for as many hours as a normal computer. It's just a different device, and all the other premium tablets on the market are OLED at this point. Except Apple.