That's just not generally true anymore. We know things about other planets in the solar system, and planets and stars elsewhere in the galaxy, and in other galaxies, without any human ever having to go to those places.

The combination of sophisticated probes and more much sensitive sensing technology has really changed the justification for human exploration, possibly so much that the justification is mostly gone.

The explorers that will go out into the solar system, like Jared Isaacman, will go for glory, fame, and/or profit, none of which can be gained by probes, no matter how sophisticated.

The explorers' dreams are of less importance than their benefactors' expectations.

Sending someone to Mars without them dying (and maybe bringing them back) will be a huge step. There's so much we can do on and around Mars, however, without anyone being out there, and some of them are also likely to be huge advancements.

Similarly for the asteroid belt. Yes, it will probably need humans out there eventually, but there's a massive, massive task of mapping and exploring it that can and almost certainly will be done without launching humans into space.

Note, I'm not saying that this would be easy or safe or cheap. But I am suggesting that the science achieved by getting some real scientists on Mars would be qualitatively and quantitatively greater than robotic missions.

So the cost with actual human scientists is never going to be below a probe and will realistically be significantly higher.

That means that the actual human scientists need to bring significant value to the endeavour for it to make sense.

At the same time we don't have the ability to reach other solar systems at all, nor projected in the next few decades. So we know relatively little about them in fact. Only as much as can be obtained by remote sensing over light years. (Which is to say: not very much.)