Huge wads of cash? Even if they had kept hacking on their own and even if they'd managed to come up with something better than the current search engines, there is no guarantee they would have made much more money.

When faced with the choice between lots of money today or to gamble that money on uncertain odds for an unknown quantity of money tomorrow, I can't really blame people who take the first option. Especially if they get to keep working on the project after having been handed the huge pile of money.

Edit: Another point of course is that actually setting up and running your own search engine requires you actually run a business and do all kinds of business things which have no relation to hacking on cool algorithms. A lot of startup founders that I know care a lot more about cool algorithms than running profitable companies, seeing the latter as a necessary evil that allows them to do the former. Those sort of people would no doubt jump at the chance of being offered a lot of of money to never have to think about the profitable company bit, and focus all their energy on the cool algorithm bits.