Some other enemies that in the main campaign you can kill multiple times for infinite score are crabs, Bubba, Tuf Boss and - in TSF - skeletons. The maximum score you can reach in the main campaign is 2'147'483'646, which is simply the maximum number the score variable can hold, except minus one, because there's nothing that could make it an odd number. To reach this value, you effectively have to score at least 105'226'698'750 points, because all objects present in the main campaign are worth some multiple of 50 points. Killing crabs is probably the fastest way to score unlimited points and yields roughly 100 points per second. This suggests that reaching the maximum score would take approximately 33 years.
And that is why we don't do it.