That's because of a history of bitcoin mining abuse.
People post their API keys on github every day and Amazon refunds almost all of the lost compute time.
I've asked and got basically a "no". I can believe that if you have a history of spending big $$$ on AWS they get increasingly flexible with raising the limits. But if you don't, they aren't going to let you spin up 500 instances, even if you ask nicely and have a reasonable justification.