In a lot of cases, people simply haven't been taught anything more than the most fundamental of English skills.
With the curriculum full to overflowing thanks to the efforts of virtually every lobby group under the sun, and the need for society to not totally write-off people simply because they lack one academic skill-set*, means that there's no emphasis on things like high-level grammar and punctuation skills.
Funnily enough, in this country at least, adult literacy rates have remained roughly stable across generations.
The perception that the Spotty Yoof are particularly inept, is largely due to simple inexperience.
*Ah yes, one of the educational wonders my generation lived through was the need to introduce communication skills into every subject - oral presentations in Maths, creative writing in Geography... All in response to the age-old whinge about how This Generation of Spotty Yoof are Sub-literate and Something Must Be Done.
Of course, this meant that if your written and/or oral skills weren't great, or you were just shy, then you'd run a very real risk of failing every subject, irrespective of how good you are at that subject...
