The root of this problem is a general lowering of standards in - probably - every area of human behaviour.
There are several reasons why young people cannot write correctly.
1. They spend too much time with computers, mobile phones, tablets, and the like. They are more used to using a keyboard than a pen.
2. Their school teachers either (a) are themselves unable to write correctly, or (b) are not allowed enough time to teach the subject. I have heard of teachers who cannot spell (and are therefore unable to teach their students how to spell), and of teachers who tell their students "you don't need to know how to spell. Your spellchecker will do it for you."
3. Students deliberately cause disruption in class. They chatter, they fiddle with their mobile phones, they throw things around. They spend more time playing about than they do in learning.
4. Young people fail to learn the basics of how to conduct themselves in society. They do not know how to begin and to carry on a conversation. They do not know how to ask a favour. They do not know how to ask for something in a shop. My next-door neighbours had a teenage son. His friends would come to the house, to see if he would go out with them. The friends would not knock at the front door, in case a parent appeared, and they had to ask if the son would go out with them. Instead of knocking at the door, they stood outside and texted him. They did not know how to formulate a simple request.
This problem is self-perpetuating. Students do not learn, because teachers cannot teach. The teachers have never learnt how to write correctly, therefore they cannot show their students how to do it. If some of their students wish to become teachers, they will, no doubt, enter college via examinations whose pass marks have been lowered to make entry easy for them. And so, those students will go on to spread the doctrine of poor writing, poor grammar and poor sentence construction. Once the rot has started, it will spread.