Modders dont have to care about compatibility/design/support/etc. They don't have to worry about the trade-offs when it comes to this stuff.
For example the local co-op thing in RE, theres a good chance that was removed not because they couldn't implement it, but just because the trade off wasn't worth it. There is *always* a trade off. The trade off here is giving access to local co-op vs having to support local co-op. It may not seem like much, but im sure enabling local co-op opens up a huge can of worms. You'd have to do more testing, youd have to do more work to make sure that it works on every machine that the game supports, and im sure there are edge cases youd run into. You also are adding just another area where bugs or other things can manifest and break the game. Basically your multiplying your work here and at the end of the day you have to weigh that against how many people want to play local co-op on PC (which isnt very many). They obviously decided it wasn't worth the upkeep on PC (which is a much more fragmented platform than say a console, so would require more work to make sure this works across many different devices), and ripped it out.
In the Skyrim place thats mostly a case of Design. The original Skyrim interface had to be designed not just for hardcore players, but for anyone who is going to play that game ever. That runs the gambit from people who are just renting a game from GameFly, to your players who have never played an RPG before, to your hardcore people who have played every thing in the series. For that kind of problem you need a design on your UI that is both functional and accessible. Once again its a trade off. They struck for a balance that maybe isnt as efficient or as detailed on the functional side in order to create a UI that was more discoverable and accessible for new players. (With the knowledge that the more advanced players would just mod it).
In both these cases the mods are "easy" because the people making the mods dont need to worry about a larger audience. They tend to develop for a much smaller audience (the # of people who install mods is very low) and also dont need to care about supporting their mods and making sure they run on a wide range of devices. Nobody gets particularly angry when a mod doesn't work.
The reality is the programming part of making anything is actually the easiest part. The hard part is designing it at a higher level and making these calls on what your willing to trade off in terms of functionality/design/support/bugs.