They are not self-sustaining, and self-sustaining colonies are what is entailed in the justification of colonizing Mars to provide a backup for Earth.

Of course, then it is illegal to “colonize” Antarctica by that definition since mining is prohibited by treaty (and many other activities are constrained by treaty as well). Not prohibited on Mars.

Cannot realistically be self-sustaining without mining.

And of course, basically no country on earth is self-sustaining, although almost any country COULD be (if your population is reduced and willing to sustain a low enough quality of life).

Additionally, a self-sustaining and independent settlement on Mars is by definition NOT a colony!

Why would we invest the resources? Mars provides a huge benefit to humanity Antarctica unless just used as a testing ground for Mars doesn't have the same benefits to justify the expense.

Yes, I agree, but strictly speaking, colonization doesn't require self-sustainment.