Thats because they dont have money! Well, that is the reason they have the ICO fund raiser. So that they can raise the money for various purposes including the marketing stages. Where as bounty is considered as part of marketing and they always allocate X % of that fund to be given to them. This way they secure rest of the money and dont get into loss even if the bounty hunters dump their token. If they were suppose to pay you in fiat then they will have to give that money from their pockets. This is because they can not exchange their raised ETH or BTC to fiat as it will break the chain of funds as soon as they convert it to fiat. They will be questioned by the banks, regulatory bodies and it would eventually will become very complicated. May be thats why they wont go that way.
Your thinking makes a lot of sense but the bounty hunters always have the worry about the future of the tokens that acquire and many times, ideas come up like this: Receive the reward for FIAT. Volatility is always in the crypto market and is a state that we do not want to be affected for a long time.
And then you need to comply with a lot of restriction and regulation by use the fiat service for your payment. you need to see that from the legal view too. It's so easy to said those projects must created a fiat rewards to the bounty participants but it needs a lot of legal requirement to the project to do that.