Because many games can be very long (my completionist Witcher 3 run clocks at 270+ hours, my last Kingmaker run was 115 hours), and having to restart because of a decision you took 50 hours ago is just absolutely terrible design.
It's especially bad when it's about decision which is by definition uninformed (as it usually happens the first time you play) and there is no way to really guess what the adequate answer ("normal" in some games is pretty easy, while in others it's pretty hard). There is also the problem with difficulty spikes (Kingmaker is a pretty good example, having to give up on a very long run because you hit one of the weird roadblock would be pretty horrible), getting used to the game so what was an adequate difficulty becomes boring, making a bad decision during the character creation that comes to haunt you a lot later, etc.