I think for the bigger mods they would have to do some QA with them, or maybe collaborate with a couple of modders to make a specific pack of mods that play well together, like a mini-DLC or something. I play Skyrim, Fallout 3 and New Vegas with a whole bunch of mods, and sometimes they have a few hiccups. For example, in my Fallout 3 my character sometimes tilts 90 degrees up or down at the waist when firing a rifle. I find it kind of hilarious, but it would bother me if I paid for all those mods and their powers combined somehow had that effect (especially knowing how difficult it can be to figure out the culprit). It's easy for inexperienced people to mod their game into a corrupted mess, and by making the mods more accessible to the general public you are going to have a lot more people angry at Steam/Bethesda because 'MY GAME IS BROKE' after installing 300 different hairstyle mods.

We tend to see our own behavior as a product of our circumstances, while we see the behavior of others as proof of their character. For example, if you see someone driving way too fast while passing you, they are aggressive or inconsiderate. Meanwhile, if you speed down a road it's because you are trying to get to a meeting on time because you woke up late this morning. Applying circumstances to other people has really helped me become less angry with the general public. Maybe that old lady is driving slow because she's got a cake in the front seat she's baked for her granddaughter's birthday. Maybe that man that was short with me on the phone really had to poop and I called at the wrong time. Maybe none of those things are true, but thinking them certainly helps reduce my anger at other people's actions.