The problem is people aren't incentivized to help, they're incentivized to get free stuff. So people who have no interest in helping are doing the bare minimum to get free stuff, which is a much larger number than the people actually wanting to help.

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.

Are you unfamiliar with the problems this has caused?

People who donate blood are pre-screened to ensure their contribution won't be harmful.

Have you been following the events of this year's Hacktoberfest?