… I didn’t say they can’t do so. I said they’re allowed not to. Since it’s allowed, that’s what they do.

Because they’re allowed not to do so. The answer is shitty yet simple.

Someone not tipping won’t change that either; all that will do is stiff a worker. This needs to be fixed by changing labor laws.

Service charge I would presume is primarily paid out to the non-wait staff at the restaurant. The kitchen in particular.
Tips go to the wait staff, and they will pay some of that out to other staff (e.g. front staff) depending on how the restaurant works.

These are going to be separate. The service charge is there so they can increase prices by a tightly controlled amount without needing to fuck up the carefully targeted price points ($8 or $7.99 is a lot better than $9.44). Which is shitty, to be clear: it’s a hidden way to increase prices while still advertising the same price. But it’s not something that replaces or complements the tip, it’s just a shitty price-adjustment.

A waiter or waitress is still going to be dependent on the actual tip.