They have different goals, Pinterest's goal is to let user create collections of things, Wikipedia's goal is to be right.

Pinterest is useful if you are looking for ideas of things that look similar. If you want to choose a pillow, just type pillow and you will find a list of collections of pillows. It doesn't make sense to show Pinterest results in Google because that's not what Google users are searching. The fact that you want to block Pinterest is just because Pinterest is sucessful and has overflowing google images has a SEO strategy.