There is a simple reason for their response, and it is this.
We live in the era of reboots.
A thirty year old game that hasn't seen a sequel in that time and has been all but forgotten is still considered a valuable property based solely on the possibility that it's I.P could be used for a reboot of it's franchise. So if they allow people to make the games free and modify them then they feel they are losing control of their intellectual property somehow, because corporate lawyers are trained to be psychotically obsessive compulsive about that sort of thing. Even if they had no plans, past, present, or future to ever revitalize an old property, the possibility, however remote, still exists that they COULD and thus they cannot allow anyone to touch it.
We live in the era of reboots.
A thirty year old game that hasn't seen a sequel in that time and has been all but forgotten is still considered a valuable property based solely on the possibility that it's I.P could be used for a reboot of it's franchise. So if they allow people to make the games free and modify them then they feel they are losing control of their intellectual property somehow, because corporate lawyers are trained to be psychotically obsessive compulsive about that sort of thing. Even if they had no plans, past, present, or future to ever revitalize an old property, the possibility, however remote, still exists that they COULD and thus they cannot allow anyone to touch it.