As someone pointed out above, anti-viruses has been screwing OS's since the dawn of time.
Which is actually preferable to being hit by ransomware or worse. At least problems with AV (and they are actually few) aren't malicious.
On-prem IT can manage this by patching non-critical boxes first to test etc
With other stuff, sure. But AV is often pushing out critical patches for 0-day exploits, and if you hang around a week for IT to try it on some spare kit you might be too late to apply it. While your IT bods are having a good play the bad guys are deconstructing it to find the hole it patches, and then hope they get to your setup before your IT people finally give the OK and think about rolling it out.
It's a matter of risk, and effectively you're outsourcing the testing and stuff to a third party who should know their onions - your local IT bods generally don't have a clue because they don't have the mindset of do-badders. Just think of how many security holes there are all over the place (requiring AV to stop them being exploited) - the developers don't have the mindset to see them, and IT support are not really any different (and if they were, you wouldn't want to be employing them).