Magic: The Gathering was designed from the beginning as a
Trading Card Game. Very importantly, it is a physical game, where the laws of physics prevent trivial copying of their cards, while the laws of copyright prevent them from stopping people selling the cards they bought.
There is also no way for someone to exploit a flaw in MTG and gain access to all of your cards to sell. Conversely, if someone exploited a bug to steal your WoW account and sell off your items for thousands of dollars, you would sue Blizzard for damages. Even worse, if you did something against their terms of service and they wanted to ban you, they would probably have to have some way to compensate you for all the items you would lose access to.
Further, WotC they actually do have problems because of this trading: they have had to create a list of cards they promise not to reprint, because of all the backlash from traders who perceived a re-print of Black Lotus as "flooding the market". Afterwards, they once wanted to still go ahead with a re-print of these cards with some special features, and again faced serious backlash and threats of lawsuits, so they backed down.