It stems from failing to teach grammar at school. And maths.
I went to an outstanding school and we were never tasught much about grammar, punctuation etc. My mum taught me all that.
The you and I thing isnt difficult to grasp. You just use the one you would use if the other person wasn't there. Ie He gave it to me/he gave it to Dave and me.
I spend much of my day correcting reports written by my team. Some of them are appalling.