Pay, Perks, People, Prestige, Projects, and just general work environment. Not necessarily in that order. Some of these are also subject to a sort of "compounding interest" for the rest of your career too that less known companies can't provide.

I'm aware things aren't perfect rainbows and unicorns there (or really, at any company). But the friends and excoworkers who've gotten into FAANG, and similar companies, absolutely love it. And not just for the pay.