@KingsixIII GTA IV was a bad port from console to pc and among many went well hated and short lived. Not a massive amount play it anymore because GTA V is out so everyone's in GTA V or in other games.
Heck so many people I know have just gotten sick of GTA V, of online and play completely different games exclusively because people get fed up of the same damn thing, especially the darn micro-transactions.
Also have you seen GTA SA:MP lately? It's still typically 50,000 active players. And people still to this day buy GTA San Andreas to play SA:MP, some because they can't afford PCs to run GTA IV or V on, others because they just like the more nostalgic experience of the 3D era games and it's zero need to be updating and online all of the time.
These things could sure be said for people that don't have an interest in using mods at all, but then when you can make the game your own, such as making it more homely by modding in vehicles, maps and all sorts of things familiar to what you see everyday at home in your country. Do I need to even say how many people like that?
Even just simple things like being a police officer, being a train driver, being a bus driver, a taxi driver, or all sorts. Roleplaying, being animals. Modding in a custom ped that maybe looks a lot like you. Changing the road signs, billboards, vehicle textures. Changing the police lights from red and blue to blues and twos. Using custom sirens. Making the game look nicer.
Not everybody is interested in all of that, maybe none of that, rather completely different things. Mods is about creativity, freedom, choice and making the game your own. Even just improving things that are already in the game as some of those above things are depending which game in the series it is.
If I didn't have mods in ATS, ETS 2 and OMSI 2, I would just have no interest in playing them as much as I do, and I sure wouldn't have the interest in doing videos of them as much as I have. But because I can put custom buses/trucks/trailers/maps in etc, I can well and truly make it my own to be unique to everything I like.
Forza, they have one very simple thing that is absolutely loved by the creative and gaming community, creating designs to your heart's desire and then other people can choose from them. That's the main thing I love about mods in GTA, I can create designs from my favourite graphics editor of choice and put them in the game. Yellow and black buses with Jasja (my wolf logo) and Wolves Travel printed on the body, oh that's my heaven. Yellow and black designed top and hoodie etc with Jasja printed on like my designed tops from my Rage On store page, yes please! Yellow and black police cars with Jasja printed on as a department logo and then printed on "Wolf Trooper", oh that's my jam right there.
Making the whole game feel more like my local area in the UK, therefore British emergency vehicles/peds/sirens, British taxies, Europeon trucks, cars, buses (or at least liveries), British signs, advertisements, maybe even script and map edits to make everything drive on the opposite sides of the roads.
Modding is wonderful and we all have different reasons to. I more so create things instead of playing the game because I love doing creative things and I'm making the game better to me at the same time.
If I couldn't do this I would be gone from GTA years upon years ago. Same with a lot of people.