We don't at present, yeah. That's a challenge. But it makes sense to have a goal to create a self-sustaining colony of humans (and plants/microbes/etc) on a different planet. Imagine that it did exist. It would be valuable to have, right?

Having such a goal and working towards it and developing the technology along the way looks like a coherent and valuable activity to me. It's not that I'm denying that it can't be done on Wednesday or in the next 5 years.

But yeah, it looks challenging.