It’s a simple answer is because humans are super squishy in space and Mars at its closest orbital proximity is about as far as we think we could actually get and not murder the astronauts. All the extra effort is just to shorten the trip a little bit because humans are that bad at being in space for extended periods of time and you’re talking about a lot of the months just in zero G transit with no rescue likely. When they do down to Mars gravity we don’t yet know how well they can function after months in zero G. It’s still an engineering problem, but most of the engineering is around, trying to figure out how to preserve humans evolved from earth in completely different conditions.

The asteroid belt may as well be a different solar system. It’s so far away humans cannot survive the journey and live launching from the moon or even Mars doesn’t change that much. The only reason it matter with Mars is because Mars is just close enough to be a viable target to send humans vs just probes and rovers.

As it stands now, most exploration will have to be done by machines and not humans because we don’t have anything close to artificial gravity.

If it weren’t for that, we could sail over there and not worry about shaving off a few months here and there. Its not like you slow down in space so you’ll get there eventually but what condition will the humans be in by then.

All the effort to shorten the trip it’s just about reducing the impacts on the humans that you hope to deliver to a new planet and at the very least say you did it. How much there is to really do on Mars is an entirely different question and we may find that our need to be there is rather limited because people only value their lives more and more and I don’t see how life on Mars will ever be anything but hostile.