Seems very low considering most users here have been on the website for a very long time and 500 isn't that much.
I mean, is it an arbitrary random number or is there a specific reason behind it?
I guess I'd make a couple arguments for keeping it at 500 points:
Ultimately the answer is that Stack Exchange decided upon 500 when they built the bounty system.
You can read more about the policies and intent behind the bounties in the help center. Reading it we can that bounties are intended to draw attention to a post.
Sites have a daily rep cap of 200. With the exception of bounties you can only gain 200 rep from upvotes each day. Over two days worth of max reputation gain is high enough to draw attention to a post without greatly disrupting the existing reputation system.