I can see how cheaper and easier can be viewed as 'not a reason not to' but the crux of the issue is that robots will do a better job than humans anyways.
Why spend more money and effort to do something less effective?
Because it is cool and inspiring. This doesn't have to be about min-maxing.
Budgets are always going to be finite, and eventually you will have to choose. Until we run out of space to explore with robots, we don't really need to send out humans.
There is no "need" with exploring. Just do both. We'll learn more about living on other planets if we actually do it.