Kids is one big reason. You can have totally different experiences before you have kids, once they arrive your outlook on life changes, risk tolerance changes etc.

If you can retire at 40 having lived your 20s/30s to the fullest then game on, but it would be crazy to sacrifice that time when you are so free and full of energy otherwise IMHO.

FWIW I am fortunate enough to have really enjoyed by earlier years and be mostly retired in my early 40s.