But why not force age verification / content restriction on Facebook / Instagram / alikes instead? There aren't really that many big players, isn't it?

Also, if what the OS does, is requiring to pick some number from 0 - 100 and date without doing any verification, everyone can lie. It has other flaws like not considering that many people can share accounts, some embedded devices with UI can no longer receive updates, etc. Honestly, if I thought for 30 minutes, I could list dozens of such problems. I doubt these laws can work efficiently enough.

For now this might sound like the least of evils, but are we sure that these idiot politicians won't come up with something even more insane after seeing the inefficiency of this?

How would you impose it on them? Facebook's only way to tell your age is for you to upload an ID document and don't we want to avoid that? But when a parent buys a device for their child, they can just enable the setting that says "this device is for a child" and then Facebook can see that setting, with no further identity information transmitted. That's better privacy, not worse.

optionality already exists and works very well

you can install very tight parental controls on many devices

but this is not about optionality, this is about forcing the mainstream into verification and certification schemes that most people won't be realistically able to avoid, it's about control and compulsion of the mainstream