because most (or at least too many) people do believe they become wiser getting older. Which is... a bit optimistic, to say the least, and at least as simplistic.
screen time is just a limit on time to learn if the kid is using it for the right purposes which you should teach them to
That too is a bit simplistic.
Screen time is (supposed to be) a teaching/control tool. Like school used to be about teaching kids limits and the value of hard work. Screen time is supposed to help kids learn there is a life outside of that phone... a very worthy motivation that completely fell flat when those kids can see adults around them waste their entire life on said phone, doing more or less stupid things like some junkies do their drug things.
But no matter how stupid adults can be around kids, the idea of giving kids some help in learning selfcontrol is great...A bit like putting safety wheels on a bike is a great idea when a kid first learn to ride a bike.
As far as "the right purpose" goes, like we would all agree that learning stuff would be one such right purpose, I would be devastated to see kids only get their knowledge through a phone. Even more so realizing kids can't focus on a single task much anymore. Here in France, the gov recently published a study revealing that something like 40+% (if not 50+, I can't recalll the exact number) kids that are reading books (on their phone or on paper) also use their phone at the same time to do other things (social, or videos like TikTok, or even play games). And then one will wonder why school results are collapsing...