I recall a flight from MDQ to AEP around 20 years ago (still in the old 737-200s) -- after doors closing the captain came on the PA to announce that rather than to AEP, we'd be flying to EZE due to "operational reasons". This is to show that they've been doing these changes for ages, although I do notice an increase since Covid.
I have never checked but I assume it's just got to do with aircraft availability. I agree it is one of the most frustrating parts of flying AR.