I'm gonna go with a really boring answer and say the devs didn't think about that too much. All the fictitional cars following the same naming schemes doesn't give the impression of several distinct brands.
I don't know if the devs ever made their mind up about this for the future, but I like the path Rockstar have taken with GTA's cars. They started with simple names like Cheetah or Sabre and by the time they spawned the HD universe, starting with GTA IV, they had come up with entire brands structured around a few different models each.
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(GTA story time
A good example of how that changed things is the name Blista. In GTA III (2001) the Blista is a bubble-shaped, Chrysler Voyager type minivan. However in GTA Vice City (set in '86) there was the Blista Compact, which resembled a boxy Honda CRX. Obviously those are very different cars, but Rockstar just went with it and those cars wouldn't interfere with each other in their respective time periods.
Then came GTA IV with a complete overhaul for the entire franchise. For some reason they decided to bring first-gen CRXs into 2007 NYC, but Chrysler Voyagers as well. Also graphics had advanced to the point where it forced them to come up with badges and letterings for each car. So they thought about brands and re-structured things.
The Blista Compact now had a manufacturer, making it the Dinka Blista Compact. They also created the Dinka Hakumai (80s Honda Accord) and Dinka Perennial (Honda Oddysey). The car formerly known as the Blista was now revived as the Vapid Minivan. And by the time GTA V launched, wherein the Blista Compact still existed, they had also introduced the Dinka Blista which resembled a then-modern Honda Civic.
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The point is we can speculate about brands and stuff, but it's not going to be anything beyond fan fiction. As someone who enjoys realism and wants to see LFS evolve, I can't help but think about the grand scheme of things. So IMO pondering about brands and models would have to take into account where this game aims to go, content-wise. Currently cars of the same class belong to different eras (80s XR, 90s RB4, 00s FXO) and too many cars' names contain Xs. TBO class in action looks more like a public track day rather than legitimate racing. So at some point there needs to be modernisation and expansion. Maybe you could get away with the XF and XR being from the same brand (for example calling them the Xeres Flimso GTi and the Xeres Ryscan GT Turbo etc.) but there'd have to be sacrifices to keep things plausible (FXO).
Speaking of which, it's interesting how in S1 the FXO looked more like a BTCC contender, and for S2 they homogenised it with the other TBOs. In hindsight I would've preferred if they went the other way, gave it slicks and turned it into a proper touring car.
Generally this kind of stuff opens a can of worms, since a lack of design was carried through years and years of development in other aspects of the game. If and when the day comes and we finally get user-created, dev-approved content, I would really like to see that stuff being able to merge seemlessly into the existing "lore". But that would require some clarification from Eric and Scawen.