Because it creates an organization that deviates from its purported mission, abusing the public trust instead of serving a common need.
The self-funded USPTO has a bias toward approving bad patents to generate revenue and consequently enables the predatory behavior of NPEs. It becomes a net detriment to society.
If it rejected 99.9% of patents, the expected value of the typical application drops to 1/1000 its current value. So fewer people would pay the application fee.
Fewer patent clerks would be needed, so their operating costs would also decrease. But presumably not below 1/1000.
If it gets a reputation for being stricter on granting patents, a lot of people won't waste their time or money in submissions that are likely to be rejected.
The self-funded USPTO has a bias toward approving bad patents to generate revenue and consequently enables the predatory behavior of NPEs. It becomes a net detriment to society.