Most people want engagement and many don't even care about the rewards much, they use hive cause it's easier to be seen here compared to the big socials. Most of those people would also like to reward the commenters but since price is low and not everyone has that much HP to get comments past the minimum threshold they can instead forfeit part of their post rewards, curators can then maybe give them a slightly bigger vote instead, etc. People shouldn't see it as a way to "lose rewards" when giving a small % of post rewards to commenters, majority of voting power is used on posts anyway.