Why Are rewards paid in ETH and not the native token to the project?
Because first, it's easier, they will exchange profits form fiat to eth and smart contracts will do the work. Second, there will be a fixed amount of REAL and even if there was not a fixed amount, paying in REAL means price for those tokens can go to a very low price because people gonna dump their profits. Just mt 2 cents
And also creating a new coins will be very hard to pump the price that's why most of the new ICO's now is using ETH as a payments for tokens you get/bought.