All your 'hard to find login' examples have one thing in common: the login buttons are in the same place. Top right corner. This is an excellent position because it is the norm. Most websites put the login button top right and most users expect the login button top right.
Your question is based on the assumption that the buttons are hard to find. I guess you aren't used to the convention yet. Next time when you search for the login button start looking in the top right corner.
I agree that other elements draw more attention then the login button. This is because most websites have more important messages to communicate. Name/logo, service description, sign up form, etc. Potential users are the targeted audience. The service needs to tell potential users who they are, what you can get and how to get it. The goal is to gain their trust and get them to sign up.
Don't be offended that the focus isn't always on the members. All members where guided to the sign up process once. Remember that most members also benefit form a large user base. So be glad the service makes it easier to sign up than to login.
Last thing, your example services keep you logged in. So there is no need to login unless you explicitly logged out. The logout button is part of the user menu and is positioned at the top right corner. The same place as the login button.