Consider the logistics.
Blood drives are conducted to gather mass quantities of blood from multiple donors, then catalog, store and distribute it as needed.
While it is considerate of you to want to donate your blood, it is not at all simple for the blood testing organization / lab / physician’s office assistants to perform the necessary collection and cataloging, and then they have to deliver your single pint of blood to the Red Cross or other collecting organization. The costs to all involved (except you) are out of proportion to the value of the donation. (In addition to the foregoing, blood collections are done with you in a supine position in case you become faint; blood testing usually occurs with you seated, and takes just a minute or two.)
Always consider logistics. It’s much easier during the blood drive to have many donors walk into the facility that is already set up to process the donation from start to finish.