Solar Paneld Roofs

Doesn't it make sense for a big box retailer to invest in a solar panel firm just to get the panles at cost and cut down on their internal use of electricity?  If Wal MArt, Home Depot, Lowe's, etc. all got together and decided to purchase a boat load of solar panels, wouldn't they get a great deal on them?  I would think at what they could buy them for they would easily save money on electric cost and help the earth all at the same time.  If they didn't want to what if they rented their rooftops to people that did want to?  Come on we just need to think smarter.