And who's covering the great expense of building these generational colony ships and training their only inhabitants, only to have them zip away never to be heard from again? (With no benefit other than believing there's a slight chance we've succeeded in making our species multi-plantary)?
(Btw, I share your intuition that the number of people willing to sign up for a one-way trip to the stars, or even just Mars, is fairly low. However, if you already look at people who are dedicated enough to become astronauts, I suspect the additional filter of asking for one-way-trip volunteers for a mission to the stars isn't all that severe. My purely speculative guess is that at least 10-20% of current astronauts would be willing to sign up.)
Astronauts seem to me to have a higher propensity for kookiness than the general population. You may be right.
For context, the most powerful chemical rockets peak around 450s-530s. A nuclear rocket of the sort we can build today would be more than twice that value, and super-efficient ion thrusters can have IspS in the tens of thousands of seconds.
But we're talking about an engine with a specific impulse measured in decades, and as far as anyone knows that means having catastrophic amounts of antimatter. I don't think plucky explorers on a one-way trip are going to have access to a small moon's worth of antimatter, and if they did, imagine how many more interesting things they could do with it than fly to nowhere?
Like what?
We're talking about a truly incomprehensible amount of energy, not just to carry the rocket and its own fuel, but the tens of thousands of kg of water and food for even a modest compliment of people.
18 years is a LONG time after all.