I'm not sure exactly what measure people use to judge students' writing. Perhaps they're listening to the news and all the test results. I'm pretty sure they aren't looking at the writing.
From what I've seen, students' grammar is no worse than the average. The same person who asks me 'why can't students write?' has probably misplaced an apostrophe and used quotation marks for "emphasis." There are errors, but most of my students at a large state school can identify them once they're pointed out. Their proofreading skills aren't well-developed, and it's possible that composing ephemeral messages on a computer screen has contributed to that lack. Proofreading is largely a question of paying attention to detail, which requires caring. If writing comes and goes, why proofread?
But writing is more than grammar. It's an ability to express ideas in such a way that others can grasp them. First-year college students struggle with this--all of them. That's because their ideas are becoming more sophisticated and they don't yet have the tools to express them. As they try to build more complex sentences and deploy new vocabulary, it's natural that they will fail sometimes. This is utterly normal.
For evidence, I share with you an article by Rebecca Osborn published in College English. She writes of faculty who wish there weren't any freshmen and sympathetically states, "There is often reason enough for the protest that freshmen are immature, confused, and occasionally downright stupid." She adds, "If our students are not supremely gifted, if their educational background is faulty, if they are not widely read, they are, nevertheless, alert young people who have, for the most part, absorbed as much as they were given to absorb." The year? 1949. Folks, in 1949 teachers thought college freshmen could be immature, confused, stupid, and unprepared.
I'm reminded of an apocryphal quote attributed to Socrates on "the youth of today." There's also this quote:
"The world is passing through troublous times. The young people of today think of nothing but themselves. They have no reverence for parents or old age. They are impatient of all restraint. They talk as if they knew everything, and what passes for wisdom with us is foolishness with them. As for the girls, they are forward, immodest and unladylike in speech, behavior and dress."That's from a sermon supposedly preached by Peter the Hermit in A.D. 1274. What I am suggesting is that all this talk of students not being able to write is cranky-old-teacher thinking. Let's focus instead on what our students need to take them to the next level.