I wouldn't call it trivial. First you have to determine where to cut it; if you cut the wrong area you have to cut again. All the steps in repairing it either take time, are messy, or require some skill, and the time adds up (e.g. waiting for the patch to dry before you can sand; waiting for the primer to dry before you can paint; etc.).

And then you have to match the surrounding paint, which is all but impossible since even if you have the same color, the original will have likely faded over the years, making your newly applied coat a mismatch, so now you have to paint the entire wall (no fun when it's a big wall). And if you had wallpaper instead of paint, good luck to you unless you saved some extra scraps.

All in all, an access panel would make the job much simpler.

The thing is you might not need to access your electric or plumbing for like 100 years. You do get a panel where access is presumably on a more regular schedule: usually the shower hookups are accessible from a closet.