Because there was fertile land on the other side.
Moving to Mars is the opposite: it is crossing the desert to go move into a frozen hellscape of radioactive desert.
> I'm sure leaving a fertile land 100 000 years ago, crossing the desert on foot
Actually, this kind of thing basically never happened, for some values of never.
Humans didn't leave Africa by crossing the Sahara desert. When we left Africa, the Sahara wasn't a desert.
Similarly, Aboriginal Australians didn't swim or canoe to the mainland, they walked there at a time when sea levels were lower and there was a land bridge to PNG.
Until very recently, nobody did crazy long crossings over uninhabitable lands or waters. Expansion happened incrementally, over habitable areas to other habitable areas.
> But eventually something pays of big time.
There is no reason to think that there is anything of value on Mars. No oil, no arable land, no liquid water, no unique unobtanium like in the movies. Just the same eighty or so elements, in roughly the same distribution. Just colder, drier, and impossibly far from everything you care about.
It's not about what you know is there. It's the potential.
There is no point for Usain Bolt to run that fast. There is not point in building the tallest tower in the world once again.
Meanwhile, the mars goal already managed to bring us spaceships that land and divide the cost of putting satellites in orbit by 10.
No other path lead to it because there was nothing to motivate it enough.
The fallout will eventually decay, but Mars will stay an irradiated hellscape forever.
> Two centuries in the future, the benefit of going to mars will be obvious IF the colonisation of Mars is successful (however you define that), the benefits of it MIGHT be obvious. Those are 2 huge conditionals; it may not be possible to colonise Mars, and if it is possible, we may still not be seeing any benefits in 200 years time.
*Of course, some did, but very much by exception. And it rarely ended well for these people, or their tribes, or humanity at large.
Why would you do that?
Two centuries in the future, the benefit of going to mars will be obvious, because we will have progressed and gain things we don't know about now.
That's how humanity works. We try crazy stuff. And a lot of it is stupid, expensive and dangerous. Plus the result is disappointing.
But eventually something pays of big time.
And everybody says of course we should have done it retrospectively.
It's like people asking why we are still making purely theoretical math research. Surely it's no use, and it's expensive.
I have no doubt many people opposed to the moon landing as well on those grounds.
I expect fully the comments to answer "but this time it's not the same because...". But it's never the same.
It's always a different situation. That's the point.
That's what discovery is about.