take a look at vexu, when teams are allowed to 3D print a lot fewer vex parts are used. Also 3D printing can be a bit scary for new teams, and if 3D printing was the norm in vrc, it could scare away a lot of new people.
Also even if a 3D printer isn’t that expensive, many schools aren’t going to want to make that investment.
And tbh it costs probably a couple hundred minimum to get a 3D printer that can make parts high quality enough to be worth using.
not to say I agree or disagree with these reasons, I just think that’s why 3D printing isn’t allowed.