It’s all about the race group and erasing everyone else’s advantages. The vast overwhelming majority of aero resistance is the rider’s body. The closer rider A can get her body to rider B’s body, the more tow.
But nobody reading this will ever need to look for the tiny advantages that road Race teams need to. Road race bikes are highly evolved for what they are and how they’re used. It’s just that using what’s good for World Tour road racing as good for bikes is totally wrong. Especially when it comes to singletrack, but in general too. Drop bars and smooth(ish) fast surface is super fun, but “civilians” are way better off with fender and big tire clearance, longer chainstays for stability and, potentially comfort. Longer reach, slack head angles, and shorter stems work great on dropbar bikes- but their seat angles won’t get much steeper because that angle works pretty good for rolling flat. TT/tri-bikes have steeper angles but are also meant to be ridden almost exclusively in the tuck+aero bars, on courses that aren’t very dynamic and surfaces that are super smooth. A great TTer’s upper body doesn’t move at all, and a mountain biker’s upper body, while it should be calm, is upright, open, and moving the bike around constantly.
Mountain bikes and bikes for non-racers don’t need/shouldn’t have short wheelbases*.
*except for BMX, slope-style, mini-velo, fixie-cross, and wall-o-deathcycles...