The fact is that placing toilet paper on the seat is a myth. It’s very difficult for bacteria to form on the smooth plastic surface of the toilet seat.
Even more so, germs just can’t thrive on clean skin. If it were so we would just get sick from holding the rails in public transport. So sitting down on a public toilet isn’t that bad for your health.
On the other hand, we have toilet paper. The soft surface of toilet paper is a ready petri dish for microorganisms to flourish on.
In addition, after being in contact with toilet paper we often touch our faces, giving the germs more chances to enter our body.