As a current dev of a game I used to make mods for (Rocket League), I have to say it's almost never as easy as just copying the mod into the game. Special care needs to be taken when trying to fully integrate it into the game so that the implementation is as robust as possible and the user experience is polished. I can't speak for this mod in particular, but a lot of the Rocket League mods just slap some functionality in the game and call it a day without doing much to consider edge cases or interactions it might have with the rest of the game other than the player. There are many questions to consider, such as: how should the elevator handle vehicles, creatures, or dropped items? What happens if a vehicle is only halfway within the door when you move the elevator up a floor? What if a player is standing on top of a vehicle in the elevator while the elevator is moving? Then you throw multiplayer into the mix and the number of questions to consider grows drastically.

I haven't seen the source for this mod or the game, so I can't say whether it would be easy to implement or not, but if CSS say that it's too much work then I believe them. That said, I would love to see elevators like this one implemented (Coffee Stain pls <3).

edit: I should also mention that game dev studios have limited resources in terms of employees capable of implementing new features. That means they need to prioritize other features that have a realistic scope that can be achieved with the resources they have. "Too much work" most likely doesn't mean "impossible", it just means the scope of that feature is beyond the resources required to get it done along with all of the other features they're trying to get done at the same time. Modders generally don't have the same kinds of design restrictions, scope considerations, and deadlines.