I'm not saying doing a CS Phd program is not worthwhile- it still is. But, if you have an existing offer at Google or YCombinator its like going to the NBA straight out of high school. Certainly there are good reasons to go to college. But at the end of the day, if the NBA is where you're trying to get to- you take the chance when the opportunity presents itself. Grad school will always be there.
also, won't google always be there?
I proposed research on how to improve upon string searching, which according to the reviewers, did not benefit society (??).
The reviewers all have PhDs, I believe. But somehow, I do not feel they are qualified to review the applications. They are looking for specific things cross the check-boxes.
The second point is that stipends are taxed. So, $3k looks like a lot, but post-taxes, it's trimmed. Also, if you look at the OP's posting, $300k for ~5 years for a student. That does not mean the student receives $60k/year for living.
The third point is that someone immediately out of undergrad at 20 might find PhD / below poverty level stipends enticing, but once you make 150k+, you really don't want to hand control of your life over to your advisor's whims.
The fourth point (which is a corollary to the third) is that I know quite a few miserable PhD students, whose advisors control large aspects of their life. For example, a few of them were house-sitting their advisor's newly purchased mansion overnight in sleeping bags. These were foreign students.