The engineering pool is broad enough (or the Bay Area shallow enough) that they can hire everywhere and get the same quality of engineer.

Since the wages paid are based on _local_ prevailing wage, and the Bay Area has the highest prevailing wage, Google will get cheaper engineers.

This also avoids the US immigration system, which is a terrible, expensive, stress inducing (performance sapping) thing for staff (and businesses) to interact with.

Finally, by distributing engineers to other countries, they counter some of the political "They're not _local_" arguments being made in various countries.