Because for me to push my own post to the Trending page is not a risk. I get to build my own brand and get my message out there.
Creating an account that upvotes itself with a leased delegation is a massive risk, one that I'd prefer someone else bankroll.
Don't you believe that excessive vote buying can harm your brand, too?
The majority of followers you generate through paid trending positions, will never start interacting with you on a regular basis, since they didn't begin to follow you for what you do (being like-minded people that have a true interest in being part of your community) but for your ranking.
You can actually do a test: publish an article and don't boost it at all. Then check how many of your ~3,800 followers really engage with you through quality comments :-)
Only organic growth is sustainable, especially when it comes to community building. That's at least my personal experience from almost two years on steemit. Even though this is a rule that applies for many other market segments.
Well, everybody has to find their own way..:-)
All the best for yours!