Title. Feels like an oversight. Just to clarify, I do NOT mean the selection of EXISTING vehicles, but the ability to choose a type of mod that is JUST a new vehicle, modded, added by players, unrelated to existing or paid planes.

(I know modded planes EXIST, just seemingly not in the workshop)

If there is some software reason for this being not possible, it sounds like a bug that should be fixed.
If there is a business reason, that ALSO sounds like a mistake that should be fixed.

The only good reason I can think of as to why this is not already a thing is time.