It appears to be so common because it's a self-selected bias of observation. The people who can't fizzbuzz are the ones who don't get hired and are continually

It appears to be so common because it's a self-selected bias of observation. The people who can't fizzbuzz are the ones who don't get hired and are continually circulating among the applicant pool. The people who can will get jobs and remove themselves from the pool. If the question is why people who can't fizzbuzz keep trying anyway, well, Dunning-Kruger. They don't know what they don't know.