Whenever I notice an "obvious" potential feature that could improve user experience in a Google product (there are MANY), I automatically assume it's because of one of two things:

a. Doing it won't get anybody a promo.

b. They've considered it and determined that it will lose revenue to a degree that is not justified by the usability improvement.

This is one of the pitfalls of having an ad-based product instead of a fee-based product. User experience is just no longer the top priority.