Android does a great job of keeping it's memory full when needed and free when needed..
Recents apps are just what the name say.. Recent used application. It's a HISTORY of your apps that were run. It doesn't mean that all apps are opened that are on that list. It's just a list of apps you run at some point in history
Android keeps some latest apps in it's memory so you can acces them much faster and if you open some new app or acces some older app from the history that is not currently in memory, Android kils the last one in history that you used and frees memory for the new app.
By not clearing recents, not using any ram cleaner apps, task killers etc, you are using and experiencing Android at it's best. Fast access and reopening apps that you use, fast task switching and multitasking between opened apps etc etc