AFAIK that's Lollipop for you. I have a custom ROM which let's me select an older style of recent apps display rather than the 5.0 one, and that does give me the "close all" button which the Lollipop one doesn't. Just as an experiment though, do you get any other options if you rotate the phone?
Of course I might ask why you want to close all apps? If an app is behaving correctly it will just be hibernating in memory when not in use, using no power, while closing it and then restarting it later will use more energy. The phone will close apps if it needs the RAM for something else, while leaving RAM empty doesn't actually do any good (this isn't Windows, with it's historically poor memory management). Hence closing all apps on a regular basis will just reduce your battery life. I honestly don't think I've used the "close all" button since 2010.