Could this be because Google and Amazon promote faster? I ask because my Microsoft offer out of college was more.
Also, the offers I heard about (from a few years ago) from both companies were comparable with respect to base salary, with negligible differences. However, Amazon's stock-based compensation offers blew Microsoft's (laughable) offer out of the water.
The point of golden handcuffs really is just that; you stick around a while and we'll give you a little extra.
(My problem with golden handcuffs is that most deals I've seen as an Engineer were pathetically small. The employer seemed to be unaware that I can buy stock options on the open market.)