I think it irritates me for the same reason I get irritated by American English speakers calling soccer “football”, or by people using a dieresis when writing “coördinate”.
Basically, it’s rare enough that it doesn’t sound natural and therefore comes off to me as an affectation, and makes the person sound weirdly smug about being “technically correct”.
I really should get over it, but like I said, not really rational.
Totally going off-topic, but for others like me who love these poetic phrases, they're called "terms of venery", dating back to the late middle ages' hunting tradition [0].
In the list I see "coalition of cheetahs" and "kindle of kittens". But then again, I don't mind new coinages, these are fun.
Basically, it’s rare enough that it doesn’t sound natural and therefore comes off to me as an affectation, and makes the person sound weirdly smug about being “technically correct”.
I really should get over it, but like I said, not really rational.