I was just wondering why Siri can't close apps, it has the ability to open/launch apps but not close them.

Pressing the home button once will "close" and app by returning you to the home screen. That is less effort than holding the home button and speaking the command via Siri.

If going back to the home screen is not what you want to do, Apple expect you to launch the app you want to be in via Siri, rather than close the current one...

If you are thinking about exiting from the app altogether (so it is completely removed from memory), that is an action Apple prefer you to do expressly using the multi-tasking bar. The belief, from an OS point of view, is that the IOS multitasking and memory management is so good that you should never need to really fully exit an app. In most cases this is true, but the odd time you need to do it, you do it manually, rather than via Siri.

Thanks for your elaborate explanation and opinion, I just wanted to know if there was a way to alleviate the four finger swipe up to not have to hit the red minus sign to completely close the application with Siri.

Apple does not prioritize closing of apps. They believe in iOS' inherent ability to manage memory on its own is fine, so not giving Siri an easy way to close apps makes perfect sense.