If humans can reach Mars, why can't they _________ [Complete the sentence.]?

Finish the sentence with something that seems like a human innovation no-brainer

For instance, my finish would be:
”...convert salt water to fresh water, economically?”

We’ve had since the beginning of time.
Imagine the consequence when human exploration/travel/transportation was largely by sea.
What if humans were consuming all the fresh water in the Western US?

Wouldn’t it be prudent to get things settled out here on Earth – war, pestilence, and famine wise – before we worry about extraterrestrial exploration?