It's because they are his kind of people, rich people, the right people. Did you read Ministry of Truth's illuminating diary on a very classy take-down of Romney by Mark from Occupy Boston/New Hampshire who questioned Romney about his "corporations are people" position? Here is the video from C-SPAN from that diary:
The question begins at 17:11, and asks Mitt to justify pro-corporate government economic policies that create enormous income disparity. Between 19:01 and 20:15 Mitt tears off a well rehearsed riff for any challenge to his "corporations are people" stance by repeating the word "people" 14 times in a one minute 14 second span. In the remainder of the town hall Mitt has little use for the the word people, or, I suppose, people themselves. But in this almost hypnotic set piece the candidate spits out the word "people" greater than once every 5 seconds.
The really funny thing is he sounds sincere, and maybe he is. With apologies to Bill Clinton, it depends on what the meaning of "people" is. When Mitt Romney thinks of people, he doesn't think about you, or your unemployed cousin, or your under employed sister in law or even me. He thinks of people like himself. It's only natural. And corporations are people just like Mitt, rich, powerful, well connected and little tasked by the society that buoys them, compared to the average American working stiff.
In the end Mitt turned his answer into a sermon preaching the virtues of the free enterprise system and castigating the failures of World Socialism citing North Korea, the former USSR and, rather oddly, Communist China, which last time I looked is beating our American ass in a number of worthwhile economic categories.
Looking for an area where Mitt has never flip flopped so as to get a compass on the guy? Here's one: Rich is Good. Think of every movie scene of a rich asshole you ever saw, looking through the help as though they were invisible and thinking of the World as an oyster dying to be slurped. Meet the real Mitt Romney.