Originally posted by Shurenai:

Currently most/all mods are external .dll files that are inserted into the game through an external third party mod loader named BepInEx. Unfortunately, the Steam Workshop does not really allow for certain kinds of external file types to be included, and as far as I know, .dll is one of them due to the huuuuge risk of abuse. (.dll's can ruin computers pretty easily.)

Not true at all, plenty of games of steam workshop mod support that distribute mods as DLLs. It's simply that the devs haven't added workshop support for them.