A very good question.

First, we caught bugs (not even regressions) just by writing tests. More importantly, many time when we left code without tests, we found a bug in it later. We're not 100% sure it's been a good investment, but we're 90% sure.

Second, because it wasn't that hard. With the tools available, writing tests that cover all of your code is not too painful.

Third, in private beta we're sure we'll have to change things as we face scaling problems and get failure reports. We wanted to have these tests before we started monkeying around with code.