Why would they sell if it's for a negligible price? Seems like a pretty terrible deal if all they get out of it is to work at Amazon.
* some return, even if negligible, for investors and the feeling of having done right by them
* decent hiring bonuses - so, some financial compensation even if the founders' equity was worthless
* reputation: they get to claim a successful exit, whatever the reality (so many founders dispense advice based on "multiple exits" which were all fire sales)
* continued life for the product... although in most cases the product is shut down and the team ends up splitting up and working on the acquirer's existing products
There's also just the psychology of it. After failing to raise the next round, you start a process to find some home for yourselves / your team / your product. Having gone through that process, if you come up with any offers, you're pretty likely to take the best one even if a purely rational analysis would suggest that you'd be better off doing something completely different.