I’m sure everyone has their own reasons, but my personal idiosyncratic one I don’t like buying apps that require online activation—I feel as though they aren’t really mine. App Store apps moved to a new computer won’t run until you connect to Apple’s servers, and consequently, you would be unable to reinstall them in a theoretical future where Apple’s servers have gone offline.

Mind you, Paddle has this problem too, but some other systems, like FastSpring, do not.

Although as an additional addendum to that—and this is probably not going to make you want to use Paddle—there is a generic keygen that works for all Paddle apps, so I don’t mind buying them because I know I have a failsafe.