In a word: Connectivity.
Why make it easy for users to log out, if your goal is to keep them on your site longer and make it easier access your "authenticated information"? If search engine results, or an online ad directs you to you a online seller then they want to make it as easy as possible for you to complete the transaction. If you're already logged in, you are 90% of the way there since the ad/search likely took you to the product detail page. An extension of this reasoning is "One-Click Buying" buttons on sites like Amazon.
Continuing the Amazon line of thinking, IF you log out: You don't have access to One-Click Buying, Saved Shopping Carts, Suggested Items, and Account information. I'm sure there are others.
This does not apply to all sites, apps, or cases, but in the example of Facebook, Google, and many other "Service" providers: They want to keep you logged in. From a user experience perspective, if you stay logged in, the things you do while away (but still logged in) from their site CAN be tracked, cataloged, measured, weighed, and turned into actionable data. With which they can build advertisement campaigns, suggest things you "Need", or just get to know the user better.
When I say away, what I mean is on other pages, collecting cookies and history. This data is available from your current and previous browsing sessions.
As an experiment, go to Amazon and look at a few items. Remember those items. Don't log out of amazon and don't clear your cache, history or cookies. Then go surf the web for a while. You will eventually come across those exact items in advertisement form. Now go back to Amazon.
A part of the experience that is often overlooked in the race to make things easier is the safety, or lack of safety, of these perma-logins. In most cases, hopefully people have secured their sensitive information by using profiles on either the operating system, the mobile device, or the browser in some cases. In the event that has not happened, the user's experience can be drastically impacted by another person accessing their logged in account. Many people, myself included, have sites that I simply don't log out. Be it from constant use, or sheer laziness, it happens.
Some sites, log you out when your session ends, or browser closes. Forcing a full login every time you visit. Is it better for security, Yes. Is it better for the user experience, typically No.