But it would also never land on Mars, so it would be a waste to build it for that. Build it for what it will actually spend its life doing.
Not saying SpaceX won't go to Mars, but if/when they do it will likely be several rocket generations later and possibly with specialized rockets, with a significant portion of it being one-time-use as you ain't returning.
Winching stuff out of said open door seems like a much, much easier task.
The outside door needn't be part of the airlock. It's certainly big enough to have an internal airlock leading to depressurized internal space.
The things that make a door that tests fine on Earth break in orbit are likely to be things that need fixing for a similar door on Mars. They won't be all the same challenges, but some will absolutely be shared.
A hatch with a winch (or two!) seems likely to be one of the smaller ones.
Wouldn't that make the mission unfeasible because it requires ISRU of return fuel?
https://newspaceeconomy.ca/2022/05/27/how-will-spacex-make-r...
(edits:) It's clearly not ideal for a short lunar landing, considered in isolation. But: what else would you do? Whatever you build, it would land on the moon perhaps once, and never again. Would you, being in charge, design a one-off vehicle for one or two moon landings—spend that R&D budget, in that way? That's not cheaper than 15 Starship launches; it's considerably costlier. (But the Apollo engineers didn't need to worry about this; it's was their express remit to spend $200 billion on one-off designs that would never be used again).
And: I hope no one suggests the "just make a unique lunar Starship variant that's simply a bit smaller". There's no "simply" resizing things in engineering. Recall that the last time Starship's length was altered by 2 meters, new mechanical resonances appeared, and it blew up three times in a row. Any "one-off" change for lunar landings is a less-tested, less-understood machine you'd be putting human lives on.