Because some people hate triangles, others hate circles, others hate squares. So in order to equally annoy everyone and therefore not annoy anyone in particular, they make them all shapes.

To make them recognizable from a distance and even if you can't read. The house shape is a crosswalk, triangles are a type of caution, diamonds indicate road conditions, and a red octagon means "stop" in every language where it appears, but some use a different shape: