It's probably the other way around. Keeping it non-free requires doing nothing, so it is the default. OTOH, freeing such a code base is costly, especially if m

It's probably the other way around. Keeping it non-free requires doing nothing, so it is the default. OTOH, freeing such a code base is costly, especially if multiple licenses cover different parts of the code or if the code contains references to stuff that you don't want to go public.