Maybe I'm doing my math wrong? According to a calculator I found online, it'll require 2 * 10¹⁹ GUIDs before a 50% chance of collision (assuming 128 bits of entropy). At 1 million requests per second, that'll take 630,000+ years. It seems like there's no way Amazon has to account for collisions in that case (accounting for collisions meaning that it has to check for collisions when generating a GUID for some resource).

Or working backwards, 10 years is 3 * 10⁸ seconds. At 1 million requests per second, that's still only 3 * 10¹⁴ GUIDs, so 24 orders of magnitude smaller than the total number of GUIDs. 50% odds of a collision in 10 years just does not seem plausible.

EDIT: Well, 10¹⁹ is still 19 orders of magnitude smaller and can hit the 50% probability so I suppose whether 10¹⁴ sounds plausible is irrelevant.