Bungie was originally going to let us equip shaders but encountered errors and bugs so decided to trash it. That’s why you can preview shaders on exotics but not equip them. As for mods, that would make most exotics too overpowered, therefore resulting in a new age of nerfing unlike ever seen before. Bungie always has a reason.
[spoiler]Ish. Reason ish. [/spoiler]
-Dredgen Qēox