Toilet paper is not made optimally.
When you wipe your butt, only that one square that makes direct contact has to be soft on the surface, the rest is just padding that gets folded in. Furthermore, only one side of that square has to be soft, nobody (I hope) uses the flip side of the same square again. In that sense, assuming everyone folds into the same side (like, half then half again, as opposed to an 'S' fold), a whole roll of toilet paper only has to be soft on one side, like how tape is only sticky on one side.
Engineering could be a problem. I can only assume that it's easier to make toilet paper the way you make regular paper, i.e. from mashed pulp, compared to making a sheet then applying something on one side of it. But if you only coat one side, it's possible to dramatically reduce the thickness of the soft part of toilet paper, and make the other side out of those hard brown towel or even bristolboard for extra structural support. In other words, we NEED to decouple the two functional components of toilet paper - soft contact and rigid support - such that prioritizing for one does not come at the cost of the other.
It's a whole nother story if you're a crumpler though. Maybe the fact that toilet paper is not designed this way is direct evidence that it's meant to be used as crumplers do.