I can't speak for all communities but there is a lot of work that goes into properly curating posts. You won't see our curators for instance voting on authors that I mentioned here that get thousands of hive per week but haven't commented in over a month or don't get any engagement. It also prevents curators from trying to do shady stuff like voting sockpuppets and too much on friends, etc since they already receive rewards for curating. Not to mention creating a list of people that shouldn't be curated for different reasons and checking they aren't on that list along with finding abusers and downvoting them. It's a lot more work than just voting for some accounts daily.