Antarctica has more fresh water than the rest of Earth too, and on top of that it's twice as warm as the Martian poles, far more accessible and even has a breathable atmosphere — but there aren't millions of people there either, and it's still considered a very harsh environment. In fact, even the people who live in Antarctica tend to avoid it in the winter.

> Also, you are incorrect in your silly assumption that life will be harsh for the millions that arrive - it seems much more likely that we could send robots to build the infrastructure while the first few land, making small cities, etc, and otherwise preparing for the mass arrival.

AFAIK, this is not literally impossible, but is way beyond our technological ability right now. If you mean a couple of centuries from now, yeah, maybe. But do we even have any proof-of-concept cities built entirely by robots on earth? (And keep in mind that to really reflect Martian conditions, they'd have to be thermally insulated cities that produce their own energy without any geothermal or hydraulic input and very little solar input and which have independent air supplies — but any cities built by robots would surprise me.)