The parts of Microsoft and Sony that do the games consoles are completely different branches of the company. For Sony they are so different they may as well have been actual different companies, not one big family. It took a good few years into the development to get the Sony TVs to even vaguely talk to the PS3, and just about as long for the BRAVIA TVs to take on the XMB and such of the playstation. And as for spreading themselves too far, half the reason they are as big as they are (and could afford to sell the playstation consoles at a loss) is because they have other business ventures to balance it out.
That's not how multiconglomerate corporations work. Yes, you are right that they can afford to take a loss in some business units because it will be balanced out by the profitable units, but profitable units will always get more attention from corporate headquarters than those operating at a loss. This means less resources, less talent, and less executive involvement for units in the red. Actually the more disparate a company's business units are, the more difficult it is for them to excel in any one area. This can be due to distraction, unforeseen market shifts, and a desire to unify processes and business procedures that might not work for different business units.
Nintendo doesn't have this problem as much as say M$, because they are focused solely on the game industry. M$ has to worry about much more than just games. It might seem like a subtlety from the outside, but it makes a huge difference in your market success. And not just for games companies, but all companies.