Simply put = if MS can sell a low-quality product to the unsuspecting consumer for a metric crapload of money, they'll do it. People involved with open-source software and do largely volunteer work often have a perfectionist streak, genuinely care about what they're doing, or actually take pride in their work, or any combination of the above. I know there's a psychological explanation for it, but people that have a sense of worth and higher purpose are more likely to put their all into developing something or doing a task than one whose only motivation is to get paid.
I know that what you said was more than likely rhetorical, but I felt like answering it anyway.