Thats not the reason. Group or solo isn't even remotely close to actual issue which is simply:
Rifts and Grifts are much shorter experience with virtually no penalty at all for not finishing/timing it, you can go again immediately (and at any level) and almost infinitely. Thus there is no pressure at all unless you push for rankings.
Remove idiotic score from M+, rankings, remove timer, make it much shorter experience and you will have the same results.