Because you pay once and that's it. Now if you did that and used it on your 3 versions out of date macOS, then an update came that only worked on current-2, yo

Because you pay once and that's it. Now if you did that and used it on your 3 versions out of date macOS, then an update came that only worked on current-2, you'd be pissed. Mostly only software with subscriptions can afford to keep supporting multiple versions.