I know this takes an extra step, but can't one simply copy the card lists from the WotC site and paste them into the box to add cards by list?
For some of the precons, of course! And that's how many people have been doing it.
Here are the cases that that method won't easily cover, though:
Multiple editions -- Some decks have their own editions (like Commander decks), and others are all predictably from the same edition (like all 60 cards in an RNA Planeswalker deck are from RNA). With these, copying the decklist and setting that edition works fine. But take the recent Guild Kits as an example: some of the cards are from the "Guild Kit" edition, while other cards in the decks are from GRN/RNA. The Challenger decks had cards from SIX editions: KLD, AER, AKH, HOU, XLN, and RIX. For these, you'll have to manually set each card's edition after import, or import the deck list in batches for each edition! Yuck.
Foils -- Even though Planeswalker decks are all from one edition, they also have a foil card (and Commander decks have 3). Importing via a decklist would entail yet one more step of finding the card that's supposed to be foil and manually changing its foil status. In some cases, these foil face cards have also been alternate art promos, thus needing a separate edition.
"Non-deck" cards -- There's also the case of the oversized commander cards, which aren't usually listed in decklists. (They are also foil and from their own edition.) Tokens fall into this category as well -- not included in the decklists, and have their own edition.
Multiple art for one card -- This most commonly applies to basic lands, which some people don't care about or track, but more recently it has affected non-basics. If you import from a decklist, you'll get e.g. 24 Islands of a single artwork / collector #, when in fact your inventory should reflect 8 Islands of 3 different artworks / collector #s. But it's not just basics. In the Guild Kits and GRN/RNA Planeswalker decks, there's 4x Guildgates in each deck, but each Guildgate has two art variants. You should import 2 of each variant, not 4 of one, which is what would happen by using a decklist.