The UK government is forcing them and all other web services to do the same. UK-hosted Mastodon servers may coast under the radar for awhile, but they'll come for them eventually.
Rather than getting mad at Bluesky for implementing government-mandated age verification, how about actually getting mad at the UK government?
Why do you think Mastodon is exempt from those rules? The law applies to all sites/services with a significant UK audience - some of the Mastodon .uk instances will definitely be subject to this, and because of how badly the law is written, it can apply to many more.
People can have their hardon for hating Bluesky, but they're literally just trying to avoid being fined by the UK govt here - this wasn't their idea.