From Adam Prosak:

Once you reach a certain point, for many effects a difference of one doesn’t have a huge impact.  For example, the card isn’t significantly stronger or weaker if it mill 14 cards or mill 12 cards.  Dodecaphobia (fear of the number 12!) is a similar card to Triskadekaphobia.  But only certain effects make sense to have the number 13 attached to them.  And, for example, “draw 13 cards” or “deal 13 damage would be dangerous to print at any cost because of effects that could “cheat” them out by bypassing the cost.

Hypothetically, development could be much more concerned about a world built around a different number.  For example, it’d be tough to do a world that was all about the number 4 and pressured us to choose that instead of 3 or 5.  But we are happy trying to find ways to make the number 13 happen.