Because most projects need to raise funds, use their own token to pay the bounty hunters as a "future money" if the project is successful, while also helping to allocate their tokens wide. Very few projects have sufficient funds to choose the payment method by BTC or ETH.
I am absolutely agree with you. ICO exists because there are projects who want to raise funds aka need money. If they are success in ICO and pay bounty hunters with their own coin then their coins will be spreading into the world.