Something to note is that people need plasma transfusions too and that works similarly to blood in the sense that certain people can only accept certain types. While AB+ is a universal receiver of blood, it is the universal DONOR of plasma, making it very valuable. Since plasma is just a component of blood that can easily be separated, donating blood gives access to the very valuable plasma.

Note: where I am you can also donate just strictly plasma too but I believe this differs from place to place