They retain them for the 4-5 years it takes for signing cash and signing stock units to all run out, at which point many people start to get itchy feet.
Well, that’s probably because he’s hit the cap on base salary. After a certain point in Amazon, the majority of your income at Amazon is derived from shares.
That said, after the signing shares are yours after the first 4-5 years, you’re down to the yearly grants they hand out, which come the year after they were granted, in quarterly amounts.
Also, if your brother is high up, he probably got more shares this year than usual, as Amazon announced that only certain levels and below were getting salary increases. Higher up only got shares.