The actual reason: child safety regulations

They don't care about child safety as long as it doesn't become so bad as to impact their revenue negatively. But they see that governments all over the world push for some kinds of age restrictions, and they know they are a prime target and it is hard for them to push back against that.

The reason they are (not so secretly) lobbying for requiring us to ID ourselves at the device level is that they don't want to be the gatekeepers. They want to make creating an account as effortless as possible and having to prove your age is a barrier that make turn off some people, including adults, and they may instead turn to services that don't require age verification. By moving the age verification in the OS, not only the responsibility shifts to the OS or hardware vendor, but it also removes the disadvantage they have against services that don't require age verification.

For a similar issue, PornHub is currently blocked in France, because they don't want to comply with the law related to age verification. Here is their argument: https://www.aylo.com/newsroom/aylo-suspends-access-to-pornhu...

If you read between the lines, you will see that they have the same stance: "put age verification at the OS level, so that people don't discriminate against us". They know they are not in a position to argue against "child safety" laws, so instead, they lobby for making it worse for everyone instead of just themselves.