Where do you want this button? Most Android apps run full screen rather than in windows, so there's no place for it. Also the Android task/memory management model is different from Windows, so there is no need to kill every app when you finish using it (it actually wastes power overall to do so, unless the app itself is problematic).
As for browser tabs, because the apps are written for small screens most don't keep the tab bar visible when viewing a page (wastes space). Some do have "x" buttons on the tabs (e.g. Boat browser), others use other methods (commonly a "tab overview" page with close buttons next to each tab). It's a matter of what the developer thinks will be most efficient as well as most familiar - remember though that there are already many people more familiar with phones or tablets than PCs!