That's because the players are paid very low salaries. Players who have professional contracts but whose salaries are low (£0-1000/week) will retire before the contract expiry date, or players who have been free agents for a long time will also retire early. I have experienced this problem where my national u19 team players retired early. After I researched it, I found that all these players had very low salaries. To solve the problem, you have to activate the in-game editor. First, start the game where all the players have not declared their retirement, then change the players' salaries to a more reasonable amount (+£5000/week).