Because DB.bnk is an encrypted file, not folder. And this file, (DB.bnk) is a group of many also encrypted .db files, like, CarPhysicsData.db, Bots.db, CarPacks.db, CarRims.db, Rims.db, CarColors.db.
DB.bnk is responsible for all the coding and information about TDU vehicles. Normally what makes the incoherence between car packs, my pack and Patch 2.00 is exactly the db.bnk. And if it has any error, the game won't even start.
To edit, or get access to CarPhysicsData.db you must do a couple of steps before.
Open TDU Modding Tools, and set the view to advanced.
Then click: File Browser
In the left part, in that list, choose Database.
Now, in the right part, a few files will show up, including DB.bnk.
Click on it. The right lower part, will be there many db files, like Bots.db, CarRims.db and CarPhysicsData.db
Select the ones you want to edit. Extract them.
Now, with TDU UT, which is a Djey's tool, drag those extracted .db files to TDUUT.exe. They should become a notepad file.
Now you have access, edit it, save, drag it again, so it backs on being a .db file.
In modding tools again, select the file you extracted, but this time you should click, replace -> Keep Name.
And voilá, if it's all right, the game should work and you successfully edited your DB.bnk. Although, if you just want to edit car physics, i recommend you to use TDU PE instead, which makes it all easier.
Edit: And that is the process i did a lot of times to add all these cars. Edited a lot of .db files.