We do! And for a long while most of the resources will come from Earth. However you can't keep doing that forever as the costs will grow to an unsustainable level.
If you could magic up a civilization on Mars today, it'd exponentially cheaper to launch materials from Mars to elsewhere in the solar system than to launch those same materials from Earth. Similarly it'd be cheaper to dig those materials out of the ground for use on Mars than it would be for them to be used directly on Mars.
Counterintuitively, it's cheaper to launch materials from Mars to Earth's Moon than it is to launch them from Earth's surface to the Moon.
If you could magic up a civilization on Mars today, it'd exponentially cheaper to launch materials from Mars to elsewhere in the solar system than to launch those same materials from Earth. Similarly it'd be cheaper to dig those materials out of the ground for use on Mars than it would be for them to be used directly on Mars.
Counterintuitively, it's cheaper to launch materials from Mars to Earth's Moon than it is to launch them from Earth's surface to the Moon.