One reason is that we don't have robots as versatile as humans.
We have machines capable of high speed, high precision, reliability and endurance not possible for a human, but no robot is good enough to look at a hole in a sheet of plastic covering the base, take a roll of duct tape and glue the hole closed within 30 seconds since the breach, unless you design and program that robot for that specific task, taking at least a year of work of a large team of engineers. And there are millions of such various easy, simple, but extremely varied tasks - a large number of them unknown, unforseen - where human intelligence, agility and dexterity allow to complete the unplanned task within seconds or minutes from occurrence of condition that forced the task.
In particular, we don't have robots that can autonomously repair themselves or each other. And this is not going to happen in at least a couple decades.
If we want to reach Mars beforehand, we will need humans to assist construction of the base. Robots can complete a large portion of the work, and about everything that was foreseen, but for the unforeseen humans will be a necessity.