because (I think) having huge amounts of attack speed both fucks with the ingame champion model (having to do another auto animation before being done with the first one) as well as makes on hit op af. imagine having, like, 5/sec attack speed, with bork/bloodrazor (not sure if possible, just hypothetical). you'd be hitting them for 10ish% of their health, 5 times a second, or about 50% of their health every second, along with the base damage. if you got a build like

And somehow got 5/sec, you'd be doing 30% current health as well as 20% max health every second, burst from the shiv that stacked ridiculously fast, and random bonus damage from nashors tooth/rageblade. and when rageblade got stacked (in a little over a second), everything would double. meaning that bloodrazor and bork are tryin to do almost your entire healthbar by themselves Woot.
basically, I think Riot was trying to dissuade the literal aids that these kinds of builds would give