The best way to learn how to write is to have read extensively. Many problems in spelling for example can be solved when with reading experience you look at a word you have spelled and see that it cannot or might be correct. Most of my spelling problems, not all of course, have been solved simply by writing the word out and then looking at it. If it is spelled incorrectly I usually see that because it is unfamiliar looking.
This is just one example. Our youth, indeed their parents, do not read. That in my unprofessional view is why they cannot write effectively or even correctly. The domination of electronic devices, television, games, etc. only contributes to the problem. And of course it also leads to speaking poorly -- without full correct sentences. Literacy begins with illiteracy and lasts a lifetime.