Every time you go trough the purchase loop you affirm the habbit of buying from them. So it was not a high priority, they probably only added it for good pr.
This is why you have to purchase games that are free instead of just adding them to your library.
It works on enough people to establish a future bussiness model. Epic game store runs on a loss. Backed by enough money to keep losing till they believe to have caught a big enough base to turn it around, stop giving handouts and jack up the prices.