Not a single one of the game's you mentioned is an Arcade Racer (except for TrackMania...I guess? But TrackMania barely even feels like a "race"). Dirt 5 is...so maybe it's for people who like Arcade Racers? I don't think the game is that hard to understand.
I don't think sim or arcade racers have a large divide. I mean, people play Rocket League, so it's not like on PC everything has to "be a sim or else." The issue is with Dirt 5, what's the identity it has? It's an offroad racing game that tries to go for a colorful extreme sports approach. But what does it actually do with it, that for example, Forza Horizon cannot do with its track editor system? I made the Trackmania comparison because it seems to be focusing heavily on creating tracks, and there's already a crazy game for that. For offroad games, it has competitors, and even better examples, within its own IP. Wreckfest, which is more of an arcadey destruction derby game, has an audience. And that's because it focuses completely on the idea of being a destruction derby game. You know from the type of game that it is, that it's going to be chaos when you race, and the game embraces that entirely. It helps it has nothing else similar in that lane.
The Dirt series has taken the core part of offroad racing and segmented that into a side series, so the main games are just "Offroad GRID." And GRID
too failed to grab an audience. I don't say this as a knock but both GRID 2019 and Dirt 5 are akin to mobile titles (not in production values so please don't take that from my comparison) in the sense that they're average and exist in a sea of more particular, standout games. They're more of titles to pick up when you're looking for something simple, like a break game between releases of interest. That means only the super hardcore of racing fans will consider those titles, and almost always on a sale as they'll likely be playing other racing games instead. Relying on potentially a super hardcore audience when you're making a more accessible racing game isn't grounds for it doing well, as we've seen. F1 and Dirt Rally are the only focused Codemasters racing games, and they're the only ones really working, business practices of greed aside. Both types of games represent very dedicated and difficult styles of racing, so even styles of racing which have actual difficulty barriers are able to find an audience. Obviously "simpler" games can too, Mario Kart is the king in the racing genre, but if you asked me what makes Dirt 5 stand out from any recent offroad racing game, the first thing I'd say is it's one of the few that has a next-gen version. That says nothing about what it
does.
If you're not a big name racing series like Gran Turismo or Forza, or a licensed game series like F1 or NASCAR, you need an identity with your game that's more than skin deep. Sadly, Dirt 5 doesn't appear to have that, and I think the lack of interest because of that shows. To make a comparison to another offroad racing game, Overpass,
I'd like to just show one gameplay sample to emphasize my point that Dirt 5's issue is a lack of actual identity beyond appearances. With a game like Overpass, you can see it's trying to craft out an identity focusing on using the physics model and methods of driving to pass challenges.
Compare that to any Dirt 5 clip and all you really see is the visual style coming to the forefront. At that point you're just hoping people rush for your game because they have new hardware and want games for it and on PC, unless I'm mistaken, the technical showcases currently for racing games is Assetto Corsa Competizione and Dirt Rally 2.0.