Because most people don't go out of their way to CHEAT on unlocking their achievements?
Just a guess.
Point went over your head. Doesn't matter so long as you can, and easily so. That automatically devalues any perceived perception of accomplishment from feats on Steam unless it's just you and yourself enjoying them, which is fine. This is also the reason why you don't see a culture similar to "platinuming games" on Steam.
I'd argue you're already cheating if your using mods anyway, especially since Larian seems to agree, but hey, keep cherrypicking, I guess. Point is that yes, mods will likely deactivate achievements like DOS2, but the mod to undo this will be available pretty quick so it doesn't matter.
And thus someone can brag they got the achievement for beating the game on hardcore tactication and honor mode on, but not tell everyone they also used modded classes and a six person party, because whoops.
