I installed the Apex full kit on my 9c and absolutely love it. I bought my gun for 350 used. put 130$ into it for trigger, and 120$ trijicon sights, and now its 100% perfect. The joy of M&P stuff is there is a ton of aftermarket support to fit your personal preference.
It's just like a car. You can go buy a $50,000 Corvette and have a sick car out the box.
or you can buy a $35,000 Subaru and put 10,000 into it, and make it faster and better than the vet could ever be. and it would be more tuned to your liking / preferences.
The thing is, do you want to invest the time or effort to customize it?
People do sights, trigger kits, barrel mods, different grips, you name it. Not because another gun "could" of had that stuff already, but because they like the gun they have and want that gun to have that stuff.
It's just a personal preference
Side Note: The apex kit with 5lb spring feels amazing.
-B