It's definitely true that startups are often founded for the sole purpose of being acquired by one of the Big Boys--but not so that the founders get to work there.

It's because they're hoping to get rich--make the perhaps $10M+ so that they can do whatever they want with the rest of their lives. You almost make it sound like the startup is part of their Google interview strategy and acquisition is how they get hired, but I'd venture that most founders probably don't want to spend more than a couple years at the parent company. They want to start the next phase of their lives.