They're devices that are highly valuable (both monetarily and how much hassle it would be to lose it due to all the 2FA tied to it) and indispensable for modern life. They're not just toys, "addiction machines", entertainment etc. - and even if they were, despite what some puritans think, there is nothing wrong with entertainment if you're sitting in a plane for hours.
Prohibiting phones on flights completely would likely be a great way to cut air travel in less than half, because it would become too much hassle to bother with for many.
Requiring them to be checked would result in massive theft, and while it would reduce the risk of fire, any fires that would happen would have a much higher risk of being catastrophic. There's a reason why power banks are required to be in the cabin.