Google doesn't own Dropbox.
Just beacuse you are able to use your Google login, instead of creating an account manually, does not mean that Google owns the website.
The login function is a free service Google offers to website owners to make it easier and faster for people to signup. It has absolutely nothing do to with ownership of the website in question.
Just about anyone with a website can use Google Sign-in:
https://developers.google.com/identity/