Why can't gay men donate blood?

I remember reading about this years ago but thought that it was commenting on a policies from the past but a friend just brought this up today that gay men are still not allowed to donate blood in the US and Canada?!

I mean if the rationale is that gay men are more likely to have a disease transmittable through blood (which I don’t agree with) shouldn’t the blood bank screen all blood for these diseases anyways?

I feel like I’m missing something here…