A couple points. Cabinet decals shouldn’t matter much because in most lineups you never see them. I generally don’t care. But I have found when I try to sell a game, if there are cabinet issues a buyer is usually going to want a discount. Whether they themselves care or not, some people do and so it affects resale value. The worse the damage and fading, the more it affects it.
As to why it affects value so much, it is a fairly big job to replace them. The cabinet needs to be taken apart, old decals removed, defects filled and smoothed, primed and sanded, new decals applied, and everything put back together. Plus while doing that it often only makes sense to replace several parts that have wear like flipper buttons, side rails, legs, maybe lockdown bar and coin door. It’s one of those things that once you take the time to make it look good it’s kinda weird not to make the rest look good.