You could but will you? It’s more work for you. It also means that right now if you do it, you could get cards they already have, so that’s fewer cards to give them.
With the proposed method they always get a card they don’t have, which ends up being many more cards for them. 2 extra cards a day doesn’t seem crazy at first, but getting 60 extra cards a month is. And this especially gets to be a lot once they finish commons and can only get rares and limiteds, or finish rares and get to limiteds, because they’ll be getting double of those despite the fact that you wouldn’t be earning just rares and limiteds if you did the dailies for yourself. We won’t stop you if you earn them and trade them away, but that’s a pretty different and less op thing. And we also don’t want users who may have less friends to get discouraged that they earn fewer cards than others while doing the same work as others