because the cryptids have special powers that blur the photos-
but seriously- they are usually taken with cameras with very small sensors, and or no stabilization in some cases which blurs the photos- not many folks carry around high end cameras but many do carry small sensor phone cameras- the person is usually excited and afraid they wont get a good shot of something spectacular or weird so they rush hte shots many times-
I also like the Futurama explanation: most people who believe in Big Foot are poor.