I've worked at several startups and you'll be surprised at why some corporations don't purchase a startup.
Some of them have awful cultures of psychological abuse, you walk in the door and you know something isn't right because the founder(s) have a cult-like environment that includes abusing his/her employees to the point where they are all afraid to say anything.
Some startups have founders and employees that are lying about everything and when you actually dig into their source code and infrastructure they aren't doing anything they say they are doing.
Some of them have the worst infrastructure imaginable and/or they have such poor software engineering practices that they will never be able to scale to meet the kind of demand a company like amazon has.
Just because they have a good idea and a company doesn't mean they have an implementation that is worth investing in.