Why do some major websites use invalid HTML?

I noticed that many websites, even Google and some banking sites, have poorly-written HTML with no quotes around the values of attributes, or using characters such as ampersands not escaped correctly in links. In other words, many use markup that would not validate.

I am curious about their reasons. HTML has simple rules and it is just mind-boggling that they don't seem to follow those rules. Or do they use programs that just spit out the code?