http://validator.w3.org/check?verbose=1&uri=http://www.google.com/ Google : This page is not Valid (no Doctype found)! Result: Failed validation, 62 Errors Yahoo This page is not Valid (no Doctype found)! Result: Failed validation, 34 Errors both of them don't have doctypes
They are coded long time ago. Changing codes for big sites like them is a huge undertaking; so I guess they skip the overhead.
What is the profit of getting those validation? Google and Yahoo will be still be the same without that validation.