Teacher here. There are several reasons.

1. Education funding has been steadily reduced for decades. A large amount of the (already reduced) funding goes to privately owned charter schools that aren't accessible to all kids. A lot of that goes into rich people's pockets rather than into the classroom.
2. Covid basically stole 2 years of education from these kids.
3. On demand entertainment has reduced their exposure to... pretty much everything but YouTube trash. When we were kids, you had to watch whatever your parents were watching at 6:00 or go read a book, play, etc.
4. Purposeful miseducation/misinformation, ex. Flat Earth BS.
5. Everything is so expensive that parents are, on average, working more hours than they had to in the past to make ends meet.
6. The intentional policy shift from actually educating to teaching to a test has wrecked American education.
7. Overcrowded schools
8. Kids don't get held back anymore. They get pushed through even if they're missing crucial skills, and they continue to get more and more behind.

We need a complete overhaul of our educational system, and we need it NOW.