The same logic applies in the military to the people who volunteer for suicide missions and hopeless last stands, and yet those have happened on a number of occasions, often to good effect, and always for a lesser cause than bringing humanity to a second planet.
I notice that you did not cite any such cases. Do you actually know of any, of you just assuming based on... what?
A "hopeless last stand" in which you find yourself facing near-certain death due to battlefield reverses (ambush, surprise attack, collapse of a defense perimeter, etc.) is not the same as a "suicide mission" in which someone who is not in imminent danger of being killed otherwise, is sent out to die on purpose. In fact, people survive "hopeless last stands" on occasion.
Genuine suicide missions are also quite rare for Western societies even in wartime. These certainly happened during WWII, with the Japanese and Soviets at least, but a high chance of dying in a mission is quite different from a mission which has no other possible outcome. Even suicide bombers have a significant chance of not dying, given the failure rate of suicide bombs.
This sort of "mission", sending volunteers to a certain death (no other possible outcome) without any necessity, in peace time, is unknown in any modern Western society as far as I am aware. Having a private organization do it makes it a criminal act.