I mean, it makes some sense if you get bitten by a snake, since its poison is not literally everywhere around you (hence why in older time they try to suck it out as much as possible). But the problem for viruses is that as soon as you stop gushing out blood, the bacteria/viruses nearby the wound will try to enter the bloodstream anyway

edit: or the fact that the veins are a one way, therefore if the virus is already inside, and you let blood out, you let out the blood which is clean. Or the fact that once inside, the virus might already be infecting cells and therefore is out of the bloodstream. or…