For me, I reskin/kitbash a lot of locomotives/rolling stock. A majority of them are freeware or from freeware origins, however, the task of trying to gain permissions from the original creator(s) as well as the individual(s) who reskin/kitbashed the original content or made components used in the my/other(s) kitbashes that I worked on is rather...well to be blunt... a pain in the backside to do. I create for my own enjoyment and because I think its neat to do so.
As far as routes go, again, similar situation. Some structures and such I edit, so trying to gain permissions from the author(s) is daunting (cause either A. Difficult to contact. B. Do not allow edits to be redistributed, even on the DLS, and C. Original creator is dead and cannot ELU becomes binding and "legally" cannot redistribute) The other issue is the routes I make are large in size and cannot be uploaded to the DLS or made into a .cdp file that is reliable to install. The 3rd issue is trying to create a list of all the dependencies that are custom made, 3rd party and their sources, DLS, and Payware and their sources....
TLDR, I am lazy and do not want to bother with trying to get permissions as well as deal with the hassel of uploading stuff and maintaining it once it becomes public.
This is a hobby of mine, not a buisness.