I think there are huge numbers of low level, new, or just... B and C level developers who come in to fill the ranks. A lot of the best devs at Amazon, when I was there, were indians who had to keep working at Amazon as it was the only way they could stay in America. I felt a bit sorry for them (and of course, from their perspective, Amazon might have been great compared to the alternative.)

This country really needs a damn visa for technologists, or something.

Anyway, at some point software development became a very popular career choice and so there is an endless supply of warm bodies graduating from colleges each year, all of whom know the Amazon brand and think working there would be really cool.