They existed at least from caveman's age, and the 3 minions that the movie focuses, were alive back then. And they were interested in serving evil leaders.
The movie doesn't explain why, so is safe to asume it was in their nature. Maybe they are the embodiment of chaos in the terrenal world. That would explain their erratic behavior, mixed language, lack of impulse control, etc....
This is the weirdest "theory" i have ever made.