During the Apollo Moon flights, astronauts have been sick: Borman, Haise, etc...
Astronauts have forgotten to throw switches (Apollo 10 when the upper LM stage separated from the lower part, Apollo 14 just before landing , with the radar not working).
Atmosphere had been leaking out of the LM, while on the Moon (Apollo 15, because an astronaut left a device open or was it Apollo 11?).

A TV camera was mistakenly pointed to the Sun and no more TV pictures of the Surveyor 3 encounter during Apollo 12.

A foot pulled the heatflow experiment loose on the Moon during Apollo 16.
An astronaut wanted to make the highest jump and he fell on his back, exposing the PLSS to a fatal rupture, but luckily it didn't happen.

Humans, even the best trained, make mistakes!!

Now imagine you're landing on Mars 6.5 months after your last serious training on the landing sequences, back on Earth; sure, you've been training during your flight to Mars, but how effective can it be?
If there's a maintenance problem on the way to Mars, it will have to pass first, ahead of training or even exercizing.
And how about the geology lessons and your geologist teacher who's on Earth and who'm you last met 7 months ago?

But robots...They cost 100 times less, don't breathe, sleep, eat, fuss, don't mind the temperatures, don't have to return back.
And they get better all the time and they shrink all the time!!
Humans won't ever shrink and won't ever stop needing the requirements to stay alive. And won't stop making mistakes because.....
mistakes are human.

Raoul