Okay hold the phone here.
1. All those highlighted stages have different layouts, which actually drastically changes gameplay for more than a sizable portion of characters. It changes who might have an advantage or not, changes how moving around the stage may behave, and ultimately Smash Bros., being a platform/fighter hybrid, ends up very much impacted by small layout differences.
2. EIGHT PLAYER SMASH IS GIGANTIC YOU CANNOT EXPECT SMALL STAGES TO WORK WITH THEM.
3. We're in the 8th generation of gaming, not the 7th.
4. Just because it was possible to mod Melee to allow 8 players on any stage, doesn't make the experience enjoyable. Going back to point 2, why would you WANT to cram 8 players on a small stage like Corneria? That actually makes for little more than a messy, chaotic cluster**** and while it might be fun at first, the enjoyment value of that can diminish greatly within a small window of time. Being restricted isn't always a bad thing, it allows you to focus your creative more on the things you are restricted to instead of having your creativity of play going in multiple different directions with no goal in mind.
That's basically the idea going on here.