There are certainly humans who would sign up to fly to Mars, even if there was no return.

Besides the fact they didn't have robots as an alternative, there are huge financial differences... a boat doesn't cost billions of dollars, and there was a potentially massive upside to discovering a new trade route or undiscovered land that could be conquered and civilized/pillaged. We already know enough about Mars to know there's probably not a lot there, it'd be like me taking a boat to Antarctica knowing there was no way to get home.