On the other hand, uninterrupted (orbital) spaceflight record now stands at 437 days, so we know humans can do it.
A Mars mission with current technology will require supporting humans for 600-1000 days with no resupply or rescue possible. The current record is well short of that and they also had regular resupplies.
The communications round trip delay makes a big difference. Ground crews are able to handle a lot of routine system monitoring and maintenance for orbital and lunar spacecraft, freeing up astronauts' time and mental energy for critical tasks. A Mars mission takes away most of this useful help from the ground.
You might say "AI will do it" and this may be true 10 years from now. But software is notoriously hard to get perfect and on this mission there are no second chances.