Because the marginal value is higher. Wander around on Mars for 10 minutes and bring back some samples and you are guaranteed to discover something new and important - same can't be said for wandering around my local park.

I love the irony of an "undiscovered gem" being "a place where a dude forgot some stuff"

One person's old trash is another person's incredibly valuable archaeological artifact...

Good luck feeling like a real explorer in a national park :-)