Could be a decent business to pay x amount for people to donate blood and sell it to hospital. People would be incentivised to donate blood, hospital would get more off it and given they are already paying for it.
Because the money incentivizes those who shouldn't donate to donate (and attempt to bypass any controls that prevent them from donating)?
That probably won't work. You want donors to voluntarily disqualify themselves when they become ineligible (like if they've started taking a prohibited mediation), and they're going to be less likely to do that if they're coming in to get paid. You can't monitor their lives 100% and I doubt it's practical to run screening tests for every problem.
To use a software analogy: it's defense in depth strategy. Their polices screen for honesty, and then they do technical screening as a further check. Neither's perfect, but together they're probably more effective than either alone.