I think DRM or any other copy protection method are implemented not to sell more copies.
I think a big studios want to have their games protected to maximise their revenue
And I think, those are mutually exclusive.
but I think that many developers prefer to have a kind of copy protection system rather than not
Some do. But I have doubts they know what they are doing. I have seen lots of threads of unknown developers trying to have drm. Amateurs. Wanting to "protect" something that most people would not play even if it were free. There is a ton of free games. Why play the drm infested one.
not to sell more copies. But to build trust to the developers and in case of the marketplace ... to give trust to it
I fail to understand how this builds trust. The reality is the opposite. You can read about examples where drm servers became unreachable and legitimate buyers had to resort to using hacked versions to play the thing they bought years ago.
Maybe you are mixing anti-cheat software with drm. Drm is stuff like having a server that checks if your offline game is installed more than once. And that server not being reachable, means, you cannot play your offline game.