because clogging and waste of good paper
I made some pictures regarding this.
Toilet paper clogs your toilet?What?What?!WHAT?!I throw tons of toilet paper in my toilets all the time, even the cardboard rolls when i'm done sometimes; they never stop it up! Then again, the cardboard rolls never really flush the first several times... gotta give it a day or two to "digest" until it's ready to flush.
Take a shower afterwards if it makes you that uncomfortable.