People also leave Google after 4 years because of the vesting cliff. When you start, you're awarded say $600k of stock over 4 years ($150k a year). Then every year you might get a refresher of another $200k that also vests over the next 4 years(50k a year).

So in year 1 you get $150k in stock

In year 2 you get $150k + $50k + appreciation

In year 3 you get $150k + $50k + $50k + appreciation. The first $50k from your refresher after y1 and the second $50k after your refresher from y2.

In year 4 you get $150k + $50k + $50k + $50k + appreciation. At this point your original $150k grant is worth a lot more due to appreciation and year 4 is when you will make the most.

In year 5 your original grant is gone so you only get your refreshers of $50k a year + appreciation. At this point you're making a lot less than last year so it's a great time to jump to Apple or Meta who will probably give you a base pay around your y4 compensation.