One possible (but not great) excuse is that there is a perverse incentive going on if the employee has any say over potential hires. If I get 20k every time on

One possible (but not great) excuse is that there is a perverse incentive going on if the employee has any say over potential hires. If I get 20k every time one of my friends gets hired (and "lose" 20k each time a normal candidate gets hired) I have a huge incentive to not give a fully honest opinion about how good someone was during an interview. Big companies can use their size as a workaround (ie, only let you refer candidates to other departments), but in startup land this incentive will probably be very real.