3 years for a man to get to and from Mars?

Just was reading on the space dot com website that it will be a three year
mission when and if we send men to Mars.

But when I was watching the animation of the Spirit and Opportunity missions
it said it took 7 months for the rover to get to Mars.

Assuming that the Astronauts spend 2 weeks on Mars That is still only 14-1/2
months round trip.

Why will it take so much longer for men to get to Mars then Machines?

Thanks.