I think there are a couple of reasons:
1. I think many potential buyers would think a car with "mods" has been ragged on more then the average car.
2. I think many potential buyers would think a car with "mods" has a greater potential for breaking down as they are unsure of the quality of the parts and/or installation of said parts.
3. If the person buying it wants to "customize" it, what fun is it if someone else already messed with it? I would want the slate clean and go from there.
