Because in TPM, we have that sarcastic battle droid. Let me refresh your memory.
Qui-Gon: I'm taking these people to Coruscant.
Sarcastic Battle Droid: Coruscant....uh...that doesn't compute....uh, wait, uhhh....YOU'RE UNDER ARREST!
I just never understood that. It would have been funnier if something like that was said by an Imperial in the OT. But how can droids even compute sarcasm?