Why do people play them? I don't really know. I can only assume certain things based on my own experiences.
I've played a number of 3D fighters against players of different skill levels, ranging from the average scrub to tournament placers and a bunch of players that fall somewhere in between. Yet somehow I still feel my experience with 3D fighters is extremely limited and I'm fairly in the dark as to why people play them or think they are fun or interesting. I don't feel I actually learned much of anything while playing them, which is probably why I don't like them at all.
Most of the 3D fighting games that I've played unfortunately boil down to extremely linear gameplay, mostly both players try to get point blank and perform moves with wonky hitboxes at wonky intervals or attempt to pin the opponents actions with high speed moves. Totally an attempt to "beat" out the player's attacks on anticipation, because apparently no one can reaction block to the high/low shenanigans or reaction counter to very fast light attacks or capitalize on footsies in general.
It certainly isn't mashing, no. But, to me it seems like a yomi game that's hard boiled and simplified. It's basically a constant barrage of Rock Paper Scissors. I mean really, I don't remember the last time I saw players actually utilizing mid-range gameplay in a 3D fighter. No baiting, no trapping, no zoning, no stagger/suki mixups off block, rarely any feinting. It really does seem like all anyone does is try to use the right hitbox at the right time -- a game of risk/reward.
Mixups aside, I don't think actual mindgames (not mixups) exist in any 3D fighters, but to be fair I haven't exactly explored them all to really know for sure.
Though I've said it before; I believe anyone can genuinely enjoy just about any game given the right circumstances regardless of how poor it is. While anything from RPS to chess might not be fun to play against a machine in, I think even something as base as just RPS can be enjoyed among friends. Especially when other circumstances are added in and things get hype. And I know we all have some friends that would much rather sit around and mash out some RPS than play a game of chess, right?
So, why am I posting this? Well, I often hear the counter-argument that 3D games "must be" more than "just-RPS" because players can do consistently well/poorly against one another so there must be more to it than random guessing. Well, that's right, but same with actual literal RPS. There's tournaments for RPS after all, and those players can be consistent too. So I'm afraid that counter-argument just doesn't sit with me. There is such a thing as being very good and consistent with educated guesses and reads. However, that doesn't mean there's any more depth to it than simply that, when you consider the rules of RPS.
So, I would personally like to hear about and see some examples of an ideal/good 3D fighter with more gameplay than "just that".