This may come as a shock, but (despite being quoted on Magic: The Gathering cards) Calvin Coolidge, Mary Shelley, and Douglas MacArthur are not in the MTG universe.
Much of the game's flavor text is made up by WotC workers. Some comes from MTG literature. And select few had a fun word as the the entire quote, such as the command “Stay” on the card Monstrous Hound.
However, many also broke out of the Magic world into our own -- especially in the game's earliest days. While sources varied, quotes bounced all around from Tennyson to Shakespeare to the Bible and Quran. On early "vanilla" card (cards that lack any sort of rules text), such as Pearled Unicorn, the only text on the card is flavor text such as (in this example) an excerpt from Lewis Carroll’s Through the Looking Glass.
As time went on, flavor text became more refined. In-universe quotes followed the story arc the game that set was going through and some even giving small hints of things yet to come. Real-world quotes also changed. They became less reliant on old-school authors and poets like Sun Tzu and the Bard and started going into labor leaders and politicians like Mother Jones, Queen Victoria, and, yes, former U.S. President Calvin Coolidge.
Seriously. Ninth Edition. Warrior’s Honor. Look it up.
By the late 2000s to early 2010s, thanks to more recent and well known people getting quotes in (Seriously, Douglas MacArthur), both WotC and players alike began feeling a bit fatigued by them. These real-world quotes largely had to come from royalty free sources for due to copyright laws.
Mark Rosewater and others at Wizards of the Coast began feeling that real world people suddenly being quoted “took players out of the game” and made it seem like sometimes it was ‘edu-tainment’. As awesome as some of the quotes really were and truly are, it just seemed to bum people out.
Early 2010s surveys also found that players preferred the in-universe quotes rather than the old ones, which led to Wizards phasing them out. After all, you can only have so many quotes from Edgar Allen Poe before you start to get depressed. (Quote the raven, "Nevermore.")
After the Core Set 2013, Wizards just plain stopped using them on Magic: The Gathering cards, bringing players back into the world through flavor text from the people, elves, creatures and whatnot, rather than a quote from Moby Dick. (This happened - Mogg Fanatic.)
Today, all flavor text quotes now are in-universe, with each quote perfectly matched or crafted to the card. However, this doesn’t rule out a future card or two having a Steven King or Teddy Roosevelt quote sometime down the line. Stranger things have happened.