Do you really have to ask "why"?
But incentivizing people to make pull requests is a different matter.
(Yes, of course, blatant spam doesn't help anyone and is disgusting, I'm talking about in general)
In your scenario, it would be like incentvizing "donating a vial" and hoping it will be filled with blood, but 99% of people are donating an empty vial.
I think the problem was not as much incentivizing pull requests but incentivizing low effort pull requests and outright spam.
The people who participate are different and pull requests are different.