> Why do we think this should be different on the internet?

Because when applied to the internet, the age gating model that works okayish for brick-and-mortar locations is ineffective (children can use a friend's older brother's ID or account, websites outside of the relevant jurisdiction can just decide not to verify age, etc.) and invasive (sending private and sensitive information off to US companies that have been breached or linked to mass surveillance). Lack of efficacy is then used to justify crack-downs on privacy tools and the need for broad content-blocking powers with no due process.

Preferable IMO would be with filtering on the local network level, like schools have been doing for decades. Parents typically own the router/mobile data plan, so it'd pretty much just be a change of defaults and maybe some new interoperability standards. A lot less invasive, and arguably more effective.