Driver import makes a list of the current host's drivers and copies only the INF, CAT & SYS files to the image. It's for basic functionality.
None of the extra files installed by the original driver package will be included.
This feature is the same as: "pnputil /export-driver"
When a full installer (like NVIDIA) runs, it registers itself under Programs & Features. NTLite can't import that entire setup.
This is why users will import the driver, have the system working in full graphics resolution, then run the full installer for the complete package.
When you say drivers aren't installed, does Device Manager see unknown devices or using generic drivers?