These guys had already done Fantastic Four/Superman Returns/The Punisher. These cameos would have had to blow the budget, not to mention getting them to do something so outside the box in minor roles.
shareFrom Edgar Wright: “Universal never really gave me any problems about casting bigger people, because in a way Michael [Cera] has starred in two $100 million-plus movies, and also a lot of the other people, though they're not the biggest names, people certainly know who they are.”
Sorta answers it. But let’s be fair, despite Routh and Jane starring in these relatively high profile superhero movies (which weren’t the greatest successes), they weren’t exactly some of the most in-demand actors.