Multiple high quality mods have been made.
I think nobody can seriously disagree, that those are a great addition to LFS?
For example currently the E-Challenger series or the Rallyecross championship would not be possible without their respective cars.
If you read some threads or discord chat then you will see how people try to get details as close as possibe.
(taking meassurements of real cars, looking at suspension diagrams etc)
So that leaves the question, what about the bad mods? Why are there airplanes and other nonsense?
I think those mods fall into several categories:
1) People are learning the tools.
For example nikopdr writes about his racing lawnmover:
https://www.lfs.net/files/vehmods/B894E9
"made as a learning practise to get used to the functions of the new LFS editor."
Such vehicle is good as first project because it is less complex than modeling a full sized car.
It is very likely that most first attempts a creating a LFS vehicle never get finished or do not get published. But if it turns out decent enough, why not publish it?
2) Experiments
An old topic in LFS forum was "A real life F1 car can drive upside down on the ceiling. Can the LFS BMW F1 do that, too?"
Once the layout-editor allowed to create a suitable track with ramps, it was tried - it worked.
So the next step is: What happens if a LFS car gets negative downforce - like an airplane?
It was only a matter of time until somebody would try it out.
How does LFS physics handle vehicles it was not meant for, like heavy trucks, busses or dragsters?
Some people just like pushing the game physics like that.
3) Mods that are basically LFS cars with small modifications:
Either again, it is people learning the tools with small projects.
Or it is just players who always wished that their favorite LFS car had a different engine or whatever.
Those mods can be filtered out by the "Tweak mods" filter.
4) For the lols.
Some users seem already familiar enough with the editor that they can create new mods without too much effort.
So they make a driveable trashbin just for amusement.
5) Low barrier to entry.
The editors are not that easy to use. But they seem userfriendly enough that many people can important some random 3D model from the internet, throw together some chasis and engine and make something that drivers.
..and so people do just that. Results may vary, some people do not really understand licenes and so on.
But it is the same story everywhere when people learn new skills. Not only in LFS but anywhere.
Millions of terrible stuff gets created daily by newbies: Horrible photoshops, awful code gets written, ear-hurting music is created. But people are proud and happy with their creations. Some will get better and eventually create content that others will enjoy too.
6) Cruisers
Some players like their cruise servers. They want as many different road-worthy vehicles as possible, physics do not matter that much. It is more important to have ambulances or cement mixer trucks for roleplay.
It is not my style of playing but really: The mods list comes with pictures. You can see that it is a delivery truck before downloading the mod. Just scroll past it.
The people who create silly mods are not trolls.
The guy who made the airplane also made a serious gokart:
https://www.lfs.net/files/vehmods?user=PedroXDBR
Look at the mods published by Jake_Blasted:
https://www.lfs.net/files/vehmods?user=Jake_Blasted
He made a tank and trucks.
And also he made the P166 and P126: Two good quality vintage racers in a quality comperable to LFS original cars.