It is because when you type with your fingers a pin, you usually leave print marks on the buttons. It is usually really easy to spot in touch screens (try changing the light or seeing it nearly in profile).
That makes too easy to discover which pin was typed, even for the most complex ones.
For example, before fingerprint readers become the norm, this trick was being used to unlock stolen smartphones and stole identities.
They are a pain for everybody, but we can't make it easy for the bad guys.