For most vaccines, the time between doses is measured in months, not weeks. Those vaccines were tested slowly over years, and the tests focused on finding the most effective timing between doses. Covid-19 vaccines were developed in a hurry, and the priority was getting them approved as quickly as reasonably possible. Testing therefore focused on the shortest time between doses that was expected to produce good enough results. Even if longer spacing was expected to be more effective, testing it would have delayed the approval by months.

Mixing vaccines also makes theoretical sense. Exposing the immune system to a more diverse range of threats may provide a more robust response when the virus mutates. Testing mixed vaccines is just more difficult in practice, because each company develops and tests its vaccines independently.