Many things are harmful for everyone, yet we only ban them for children because banning them for everyone would create huge uproar. Make tobacco illegal and then we can talk about social media.
Not really. The US has widely legalized gambling of various forms (sports, prediction markets, etc) in recent years. I'd argue that even more so than nicotine there is zero benefit to gambling.
Tobacco is legal because of large numbers of addicts which would create a huge market for illegal supplies. The UK plans to phase out tobacco by raising the age at which it can be legally bought every year.
In any case, this is whataboutery. There are lots of harmful things that adults are allowed, but lots that are banned for everyone. There are lots of things that cannot be sold without meeting safety standards (cars, electricals), or that are only sold subject strict controls (prescription drugs, guns). We do not allow adults to have whatever they want.