It can be, because unless the game comes with all its own assets, it can't be a friction-free install for the end-users.
You can't expect to get the success of OpenTTD if you're unable to put the game on steam, and you cannot put it on steam if it won't run without the original game (which won't be on steam).
There other question of successfully cloning a game is "Will we ever be able to replace the requirement of needing the original game's files?"
With 2D games the answer is almost always "Yes, we can sooner or later have replacement 2D assets".
With 3D games you can forget about it; the effort and skill level is too high for volunteers to make it happen.