It's been a while since I last donated blood, but last time I checked some conditions are hard to reliably check for especially in the earlier stages. For example, if I remember correct HIV is hard to reliably detect for the first ~6 months or so, as is Hepatitis B and some other conditions.

This is why you get a questionary every time asking whether you had unprotected sex, recently had a tattoo, etc.

Those tests are faster now, however what remains entirely untested is simple sickness like colds. That's why you're supposed to call them if you get a cold within a few days of donating, they have no other way of knowing.