Why do people wipe feces on walls?

There are a number of reasons why someone might smear fecal matter on the wall.

Sometimes, in the case of children, they don't know any better.

Some do it when they are lonely and bored. For instance, if you had to spend years in solitary confinement without any radio, television, books, or Board Games, you may be tempted to play with your body waste to have something to do.

Then there is mental illness or senility. The person may be back to a child-like state, or they may have delusions or not know what they are doing. Some may think of themselves as artists, and think of the fecal matter as brown paint. So the wall becomes their canvas.