Why separate?
Investors might feel they'd getting ROI if their investment was purely in high growth cloud services rather than the slower growing retail behemoth that's also part of the company but the pricing already factors that in and there's a key factor in AWS' success: They know it works because they use it.
I can't see value for existing shareholders in spinning it off, even if the spun off company is attractive to new ones. The retail arm's drop in value (due to now having increased friction and expense in its IT hosting) would offset any direct gains for existing shareholders.
Possibly more relevant, because I'm not CEO material, if there was value in spinning it off then a certain Mr Bezos would already have taken that route - he's demonstrated a level of business building prowess that, while not infallible, I'm not sure greedy investors can pretend to match.