Why didn't CD Projekt include this?
It would obiously be optional and those who don't want to play a NG+, don't have to.

Obviously I can't speak for them, but one would think that it would ruin the second act. The second part of the game is totally different depending on your choices, so if they incorporated a NG+ you'd be able to go through that completely different Act 2 for the first time as a god, and the entire experience would be trivialized as a result.

But this wouldn't impact anyone who doesn't want to use the feature, now would it?
Personally I'd use it only my third playthrough, but why should you force a certain playstyle onto the players?

By that logic, why not have an optional god mode programmed into every game that can be accessed on a first playthrough? Not everyone would use it, so why not?

Not having a NG+ isn't forcing you into a certain playstyle. It's forcing you to have a playstyle of your own rather than allowing you to walk into every situation knowing that you're an unstoppable juggernaut who doesn't have to plan or think ahead.

Besides, there are probably trainers out there that can give you stats and stuff like you have at the end of the game.

Yes, why not?
Why can't everyone play just how they want to?

What adds to this problem is that on the 360 there is no way to use trainers or mods.

I wouldn't mind having a new game +. If you don't want to play a game + then don't play it.