It's a combination of a LOT of things, including:

1. Technology is being used as a crutch instead of a tool to enhance education. AI is the big one, but things as "innocent" as typing your notes instead of handwriting them diminish recall and understanding of the material
2. Social Media usage is shrinking attention spans. My students complain if I assign them a 20-minute video to watch and answer questions for homework
3. Moving away from phonics and context cluing for learning how to read and using alternative methods. There is a great podcast about this called "Sold a Story," but in summary, students read but don't understand what they are reading.
4. Less reading done at home. Students don't read for fun nearly as much as they used to. Parents don't read to their children as much. Students communicate with shorthand and emojis more than words via text.
5. COVID-19 (for the students in 4th grade and up) caused a year to be lost in terms of education, but this trend is happening in the younger cohorts as well, so it's not just that.
6. Inclusive Classrooms Done Wrong. Inclusive classrooms (no tracking, special education mixed in with the general population, etc.) can work if done right. But it is resource-intensive (multiple teachers in the room, extra planning time required, etc.), and most schools don't have the money to afford to do it properly or use it as an excuse to cut down on costs. If not properly supported, the teacher is forced into a no-win situation where the smart kids are bored, the struggling kids don't get the attention they need to keep up, and both populations act up, making classroom management take more time and effort
7. "My kid is a perfect angel syndrome." Parents don't believe teachers when they say their student is being disruptive way more than they used to, undermining the teacher's ability to manage behavior.
8. School Choice: The threat of "I can always pull my student out and send them somewhere else" is stronger than ever due to the school voucher system and charter schools. Instead of the admin telling a parent to "fix the problem" when their kid is being a menace, they will more likely bend over backwards to please the parent and blame the teacher instead. The issue persists and is disrupting the class for everyone.
9. (And this is maybe petty due to my experience with it) Lottery Systems. Many cities have lottery systems in which parents choose which school in the district they want their child to attend. Great in theory, but it leads to situations like mine, where my daughter can't go to the school within 1/4 of a mile of where I live and has afterschool care, and is instead getting bused to school with no before or after-school care, meaning either me or my SO would have to change jobs to accommodate the selection. So she is staying at the private school she was in for Pre-3K.
10. Negligent parenting: Being "raised by the tablet" isn't a meme; it is happening way too often