One problem is that you can't really copyright game mechanics, at least in the United States. Games are still protected by IP laws in many ways but it's pretty easy to write a direct clone from scratch without infringing anything.
You can actually patent them[1]:
A patent was granted to Wizards of the Coast in 1997 for "a novel method
of game play and game components that in one embodiment are in the
form of trading cards" that includes claims covering games whose rules
include many of Magic's elements in combination, including concepts such
as changing orientation of a game component to indicate use (referred to
in the Magic and Vampire: The Eternal Struggle rules as "tapping") and
constructing a deck by selecting cards from a larger pool.[9] The patent
has aroused criticism from some observers, who believe some of its
claims to be invalid.[10] In 2003, the patent was an element of a larger
legal dispute between Wizards of the Coast and Nintendo, regarding trade
secrets related to Nintendo's Pokémon Trading Card Game. The legal
action was settled out of court, and its terms were not disclosed.[11]