Because Apple hates their customers that's why. Need I remind you of the Genius Bar memes of customers being pushed to buy new devices when there's a minor fixable issue?
Okay for starters, there can be defects in the product not expected. Youtuber DankPods bought a Apple StudiNo Display and after two months of use a single firmware update bricked it. After that, he decided he didn't want the monitor anymore and the headaches it caused him. Since he bought it from a third party the third party had a much longer return window and gave him a full refund.
If this was the Apple Store, he'd be stuck with that monitor he disliked. 2500 Aussie Dollarydoos down the drain.
Secondly, why are you even questioning that? A longer return window is extremely pro consumer and painting that as a non-issue is nonsensical.
I seriously doubt the amount of abusers would increase dramatically if the return policy was increased at the Apple Store. There would be abusers sure, but the amount of good customers would far outweigh the abusers.