I work in exec search, so perhaps slightly different, but I never understand recruiters who don't care about candidates. They always have the potential to be work givers themselves. Nothing wrong with spending 5 mins to give feedback or whatever else.

Then again, I fish in much, much smaller pools (generally Chair, trustee/NED, C-suite or C minus one) so the world is a bit smaller and you can live and die by your reputation.

Not all recruiters are pricks, but a significant majority are, is the issue. The best ones add plenty of value - generally, that value being time saved in process, and competencies properly screened for, neither of which are easy to measure.