Not having an option to choose an arbitrary amount as bounty is purely because of historical reasons. When the bounty system was first introduced back in 2009, there was a slider to choose the bounty amount with increments of 50 reputation.
The reason for not having an arbitrary increment seems to me that it made the classification of different bounties easier.
And with the improvement of the bounty system in 2010, this feature was carried forward.
I don't think that it would be difficult to implement such a thing now but the only problem would be that we'll start seeing random numbers as bounty amounts not visually appealing (or may be it is just an acquaintance with the ongoing normal). However, the 500 bounty limit is justified, so as to not give a user, not well acquainted with the system, a large amount of privileges suddenly.