Someone was sloppy once, got sick and sued, they did a bunch of tests and figured that normal germs won't be possible below 5 and to cover their asses now recommend 4.

Blame the litigious system we have, same reason every plastic bag says don't put it over your head.

If you're smart about it, use the vial.

If you reuse needles to extract, use giant needles to core the hole and ruin the seal, or just keep it in filthy conditions, then that's your fault for getting sick.

Another thing to consider, some... Not all... But some are filled with inert gas that prevents oxidation or other reactions that can happen when exposed to the air.

If enough outside air gets in, there is a shot that has a reaction with the contents and you're now not getting the dose or quality you need, then wonder what's wrong.

Chemistry is a bitch, be careful and you should be ok, but just know what you're risking and check if it reacts to general outside air, or if it's really a germ contamination thing.