The cards _could_ be copyrightable, would probably be essentially a coin flip if you took it to court.

No individual card text (limited to just the mechanics) is copyrightable but the setlist of cards might be. It would come down to how much creativity went into curating the list of cards that is released. It gets especially murky because new cards are always being released and old cards are being retired, so they obviously put a lot of creative energy into that process. You'd have to avoid pre-made decks as well.

Unless you have funding from an eccentric MTG-loving billionaire, I see why you'd comply with the cease-and-desist.

Yep, plus you've got to worry about the card names (unless you're giving every single card a new name like Wizards did with "Through the Omenpaths") and whether a judge thinks that "no we don't distribute the images, we just have a big button to download them all from a third party!" is a meaningful distinction or a fig-leaf.