Will items from http://www.3dcadbrowser.com not work in game? Would be nice if they would just be able to download and put in the folder.
Most of those models are paid, and a big part of those aren't accurate. Also, they got no frame, undercarriage or mechanical parts modelled.
Most are too high poly for our purposes. They don't have any of the frame, suspension, drivetrain etc modelled which you need. You certainly can't just drag and drop them as they don't even have the node/beam work done on them.
It still has a long way to go and has many planned official vehicles plus the growing number of add-on vehicles people are making.