Because that's not how public health works, it's how anti-vax spin works.
Yes, fully vaccinated people can still catch and pass on the virus.
But they're about 80% less likely to catch the virus, and those that do are about 50% less likely to spread it.
So about 90% less spread amongst the fully vaccinated population - which makes "but they can still spread it, so why bother" a completely bullshit, argument.
a dangerously bullshit argument.
On top of that, it's not just about spread, as it's become inevitable that everyone will get covid at some point now; it's also about managing severity and keeping people out of hospital beds (where their personal decisions end up killing someone else who can't use that bed) and morgues (because even the unvaccinated deserve to live)
Of course risk doesn't just go away - the only that happens is with an elimination strategy - that's been made impossible by anti-mask, anti-vaxx, anti-lockdown sociopaths; and their pro-vaxx spreaders of misinformation (who aren't sociopaths, just misinformed)
ETA: the 80% and 50% are pulled from memory; those numbers do stack, but only partially, as far as I've seen no-one knows how far they stack, so the real number will be somewhere between 50% and 80% (assuming I've got those right), but even if it's a 50% reduction - that's still massively worth it, and it's a 50% reduction for each and every interaction. 50% is a hell of a lot better than, for example, a bike helmet