Because "Microsoft", as a brand, is recognized as a corporation. "Google", as a brand, is a search engine. People don't want to use corporate branded things daily. It's why the IM is called MSN Messenger and not Microsoft Messenger.
On a branding level, some things work better if corporate branded, some if independetly branded. You can't just throw the same brand identity at everything.
Imagine for example using "Procter & Gamble Soft Soap", "Procter & Gamble cleaning fluid", etc.
Apple is a brand for consumer electronics, and that's where you see their brand. There is no 'Apple' search engine, for example. Their OS for example is not branded Apple OS, it's branded as OS X or Mac OS X.
On a branding level, some things work better if corporate branded, some if independetly branded. You can't just throw the same brand identity at everything.
Imagine for example using "Procter & Gamble Soft Soap", "Procter & Gamble cleaning fluid", etc.