I am a layman when it comes to this stuff, but I don't understand why we don't build a large ship like the ISS in orbit. Then we can launch the propulsion portion separately and attach to the cargo and living areas. Leave the whole mess in orbit around Mars on arrival and descend in a vehicle solely built to get crew to the surface and back. Is this not doable? It would solve a lot of the issues with the long voyage.
There is considerable disagreement on the topic.
The Mars Direct initiative is probably the most famous in advocating for sending humans to Mars in smaller, Earth-built craft. Since you ask why not, I'll focus on their reasons (rather than those why we should build craft in space, or why we shouldn't go at all). The argument is naturally long but I'll point out:
- Proponents feel that orbital construction will never finish, as political winds and budget cuts will inevitably get the project cancelled.
- The engineering challenges in constructing a large, complex craft in orbit have yet to be overcome. Mars Direct could be achieved with existing technology.
- Estimated cost is much lower for a Mars Direct-style mission.
As I understand it, building interplanetary ships in orbit is the long term plan. Building the individual pieces is the hard part. Building a propulsion system that meets the mission requirements, building the long term, deep space human habitation module, and building the cargo section, for example, are the parts that we're having problems with. Especially the part where you have to keep the humans alive. Not to mention that building and launching each individual pieces is really expensive.
You might find these questions interesting relating to the individual pieces: