For having so many small experiments and projects to maintain, a human presence is really not that much more effort compared to building robotic versions of each experiment. The human is also far more adaptable, able to repair and rebuild systems as needed.
Well, except that humans are pretty much stuck at the landing site. Mars has half the circumference of earth or about 20000 km, you can get the equivalent of the lunar rover and cover maybe 20 km before you have to turn back. Sure, the rovers are a snooze feast but we got several of them in different places. For the same reason it's not practical to repair them or return samples to base either, even if we had a man on Mars.