There will always be a percentage who didn't pay attention in school or are just not very bright.
Would be interesting to see if there is any correlation in the age of the more common offenders (I'm 34) - at the time I was supposed to be being taught the basic fundamentals of English was the previous time the Conservatives were in power and as they like to do having a good old meddle with the school system - I dunno how widespread it is but around here a good number of people around my age lack a proper English education (they kept changing around the years certain things were taught in the curriculum so 1-2 years fell through the cracks) - I was never even taught about things like nouns, adverbs, etc. at school due to it (wasn't just me not paying attention or my school being a particularly bad one),