When you initially start at Google, say, you get an RSU grant that vests over 4 years. So for 4 years you are getting your salary and on some schedule also getting stock. The stock can be a quite large portion of your total compensation (as in, comparable to the base salary).

After 4 years, unless you got refresher grants, your compensation is just your salary, so you effectively make less money than during the first 4 years. At that point the incentive is to move to some other company and start the 4-year clock again...