This is because by putting several cards into one's own graveyard, they can easily be accessed through the use of reanimation and recursion spells
Having unlimited access to repeated cheap life gain can turn on dozens of powerful payoff cards
The Second Rule: 2004 to 2013 The second iteration of the legend rule, introduced alongside the release of Champions of Kamigawa , required all copies of a legendary card to be moved to the graveyard should more than one copy appear in play
A cheap card like Sleep Magic is a good way to stall powerful opposing creatures, or you can eliminate them with something like Overkill
(I should also mention that this doesn't include reprints, so fetch lands are counted as Zendikar cards, not Modern Horizons 2 cards, so they aren't skewing the data.) While Modern Horizons sets add a lot of interesting new cards to the Modern format, at this point, it's clear that they also add a lot of expensive cards into the Modern format, which in turn ends up increasing the cost of playing the format
If you want to build a deck that tries to incorporate this mechanic, there are still a few ways to go about it