I don't have an official source for this, but I believe it is to make sure everybody always has at least those cards, and is able to build decks with which to play.
In the distant past, there were a few cases where players, not realizing what would happen, milled so many cards that they were unable to build a single deck -- in other words, it became impossible for them to play the game.
Or, they milled entire factions and were then locked out of those factions until they crafted or opened (from kegs) cards for the factions.
In any case, the reason most certainly is not to get players to spend money.