The thing with age verification is that it's a legal requirement, so they're all going to have to do it.

Running an online service with a lot of users is very expensive, too. All these proprietary services eventually realise that they need revenue in order to continue operating. That's one of the reasons why decentralized, federated services are the way to go - everyone runs their own small community, so there's no one company or person that has to cover an enormous bill.