Because a Villain questline in 99% of games is just the Hero's questline, but sometimes you stab someone's dog or grab the evil sword instead of leaving it there. There's no room for substance or content because the linear narrative specifically requires you do things in order to reach the pre-determined later levels, and they rarely write the original non-villainous story in such a way where a villainous approach will lead you to the same narrow spaces that get you to "the end".

In the games where you actually have a full story and narrative behind the evil route, there's a chance for everything to be completely different, full of more engaging content, a rich narrative, different playable characters, access to different tools, etc., but these routes 99% of the time also require a measure of context as to why you'd be evil in a world where your general pull is towards heroism. This then usually means they need to feed you a full narrative before you can consider why being evil would make sense, which then generally means you're not going to even know why you'd be evil or feel the impact of being evil until a full non-evil playthrough, which then means being evil is purely supplemental content, rather than primary content. That, or you make the game with the evil route as the intended playthrough, in which case it's a game where you're supposed to be evil in the first place, with the "good" run just being the introductory content to kick things off. You can't have perfect balance without sacrificing some element of design, so one of them tends to usually be lopsided as a result.