Google (and most large established tech companies) give RSUs instead of options, so you don’t have to spend any money to exercise them, but your point about vesting is good.
This all the more reason employees dont "just quit" and join another company. Say they quit and join Amazon -- now they are stuck on a 15/15/30/40 vesting plan and just kicked all their comp into a future year that may or may not arrive. Say they quit and join a typical startup, now they have a 1yr vest.