Free downvotes will give power to random users that hasn't earned Stake for a long period of time. It also gives less incentive to hold Steem Power as it wouldn't give you power.
The core thing is people want to advertise. And Steem currently doesn't provide a good service for this at the moment. This is not the users fault. Human behaviour will be as it always has been. So it's not that the people are evil. It's just that they are using what is being offered to them. As the supply of manual curators are small. People realize it's best to earn an ROI by themselves.
Also promotion stuff barely make any big ROI. You have to offer real value and get organic support to make it effective. Low effort content is just the nature of humans. Most people have no clue how to create high effort content. That is just how life works competitive wise. But competition wise people that offer low effort stuff won't win out in the long run. As it will become more competitive as time goes on.
It's expensive to create good content. So it needs a constant flow of massive value coming in.