Why do they scan anything on folder-open anyway? Shouldn't it just do that on file creation/modification events?

Presumbly there may have one time been some type of malware that executed through thumbnails with embedded data or some other type of convoluted nonsense that the scanners had to protect against. I think I'd rather take the risk of that though over the accumulated production loss of watching explorer hang for 5 seconds everytime I open a large folder

That's accurate. And I will say that I've seen it hang for as long as fifteen minutes opening up my Downloads folder - which isn't all that big really.