Like hedge funds, private equity funds charge high management fees as there is potential for strong gains, but it's also high risk. If private equity investors find an existing company that they believe is underperforming and they buy it with the intention of turning it around, there is a lot of risk involved with this (and yes, research and due diligence). They might not truly understand the company's problems until they buy it and look under the hood.
It might seem like it is just a matter of firing incompetent old executives who are making poor, outdated decisions and don't have the innovation or insight to grow the company, or cutting out a whole lot of dead wood and replacing employees with automation, or outsourcing certain parts of the company to decrease expenses. But there could be so much more to it than this. It could be a total lemon and require a lot more work, coaching and management than they expected. What if all the employees are so demotivated the private equity investors have to completely restructure the whole company and pay out redundancy pay to a lot of people?
They have to charge high fees for all the work they put in before they can wrap the company up in shiny new wrapping paper and sell it on for a big profit (hopefully) for their investors.