Why? (No Brexit questions allowed, please!)

This could be a thread where the great unanswerables of life could be aired. The following one occurred to me at lunchtime.

Why does peanut butter so often come in those silly jars with a shoulder in which the stuff gets trapped, making it impossible to get it out without a small teaspoon or your finger? The world contains many other versions of glass jar - the Bonne Maman jam jars would be perfect - wide at the top and narrow at the bottom.