Epic cannot do that because

In response to one inquiry from a game publisher, in another example, Valve explained: “We basically see any selling of the game on PC, Steam key or not, as a part of the same shared PC market- so even if you weren’t using Steam keys, we’d just choose to stop selling a game if it was always running discounts of 75% off on one store but 50% off on ours. . . .”

(source)

However, Epic regularly offers coupons out of pocket. Right now you can essentially get 43% off any game above $14.99 or the regional equivalents, as many times as you want, even if the game is already discounted by the publisher.