"Shouldn't" is a bit strong. The reasoning goes that as the front wheel carries less load than the rear, then having the same spoke count front and back means that either your front wheel is heavier(and stronger) than it needs to be, or your rear wheel is weaker than it should be.
It's a question of optimization(or lack thereof). In practical terms the drawback to having a few spokes more than absolutely required in the front is negligible outside race conditions.

Wheelbuilding is fun, but that is usually false economy. As a rule buying the parts separately is more expensive than buying complete wheels.

Heavy rider, fast and hard, looking for durability - That's a good reason to look for 2.0/1.5/2.0 NDS spokes and 2.0/1.8/2.0 DS spokes. Maybe a 32H front and a 36H rear.