They do not read books or other printed materials.
Instead, they read each other's posts on SM, along with very poorly captioned reels, etc. They absorb the ubiquitous mistakes and in turn spread the poor spelling and grammar themselves.
I suspect they never had correct contractions drilled into them, or proper use of its and it's, or there, their, and they're, or how to use an apostrophe for the possessive case. But the big problem is lack of reading.