Are DPEs incentivized to fail people on checkrides so they can collect extra money on re-tests? I'm almost ready for my private pilot checkride, and it seems to me there is a perverse incentive for DPEs in passing or failing people.

If a DPE makes additional money on re-checks for students who fail the checkride, then it seems to me that DPEs have have a strong financial incentive to fail people the first time so that they can collect additional fees on the re-test. In fact, it seems that there are no rules in place to stop a DPE from making a student do the checkride 4, 5, or 6 times and collecting additional money from the student every time.

The checkride fee is exorbitant ($900 here in Southern California), but I'm even more concerned that the DPE would be hoping to fail me so they could collect extra money from me on a re-check. It actually makes my blood boil just thinking about it.

I'm not saying that DPEs are bad people, but the incentive structure certainly seems to make it easy for them to exploit eager students, and if economics has taught us anything, it has taught us that people respond to incentives.
I think you are getting worked up about something that isn't a problem.  DPEs are incentivized to develop a professional reputation that upholds appropriate standards, while passing those who meet the standard.  Someone who is motivated by greed to fail students as you suggest would quickly get a bad reputation and people would simply stop scheduling check rides with him/her, so the plan would rapidly backfire.

Think about it, I could easily make the opposite case that I am concerned about safety because a DPE motivated by the desire to get a reputation of passing people the first time to increase his/her business could be tempted to compromise on the standards.  It should all balance out in the end.