That's the only real answer. Like most baddies they can slaughter extras by the thousands.

Hardly surprising that stormtroopers rarely hit anything. They’re using the notoriously short-ranged Sterling SMGs with the butt folded (and no magazine).

Except for their section support weapon guys. They have infantry-pattern Lewis guns, again with no magazine (check out the scene where Luke and Ben arrive in Mos Eisley in Episode IV).

Hypothetical battle:

An army of Star Wars stormtroopers vs. an army of the Padishah emperor's Sardaukaur? The latter is from the Dune universe in case you didn't know.

On the topic of Empire troopers not hitting anything with their guns: