They won’t vent nearly fast enough to prevent crankcase pressure from building up. Again, they’re not intended to vent gases out, just to let air in.
If you’re referring to the old engines, pre-PCV, they had a draft tube that ran down along the engine low enough to catch the air passing by the end of the tube as you drove which caused a drafting effect that pulled the gases out from the crankcase. Hence the name “draft tube.” So even those were much more than just a vent.
