Digital

I'm not just being cynical when I say that the point is that Wizards won't make enough money if your collection fills up too efficiently. I guess they might say something like "we want the experience of opening digital packs to closely mirror paper". But if you pursue this line of questioning you pretty quickly end up at "why doesn't everybody just get four free copies of every card so personal wealth isn't a factor in deckbuilding?" And the answer is that that's not WotC's business model. They have to make money -- even if we set aside the matter of profit, they have to fund maintenance & development somehow -- and selling you packs that aren't guaranteed to contain anything you need is part of how they stay afloat.

Maybe the more pertinent question is "how do they justify selling you things they know you don't need?" If we're asking about Arena, duplicate commons and uncommons contribute to your Vault progress, and periodically you'll get to open your Vault and get six extra wildcards. It's not much, but it's not nothing. If we're asking about MTGO, theoretically you can trade your excess cards or sell them to bots, though typically for less than a penny apiece.