There's so many different coaches all over the world they can't all be making the same mistake.surely some of this falls at the feet of their coaches, no? Can't be taught a plan b or c, or other dimensions to their games?
I my opinion there are 2 main reasons.
1. Tennis is getting a worse and worse alternative for young athletic talent. Sports like football and other team sports are getting increasingly viable at lower and lower levels as the money is going up there. Meanwhile, even though tennis is getting more and more money, it still can only really sustain ~150 players on the ATP or WTA
2. As the senior game has changed, the junior game has changed with it. Poly strings enabled much more topspin and killed serve&volley, but I'm also convinced rackets with bigger sweetspots where you can just muscle a ball as long as you hit enough spin absolutely killed the developments of juniors. Almost all young players have major technical issues the older dudes didn't really have.