Disable Screenshot Security Some apps don't allow screenshots so how can I still take a screenshot?

It is my phone and I should be able to do whatever I want with it so why is there even a feature to disallow screenshots?

Android 5.0.1
Why apps prevent screen capturing, and how to disable the restriction
Some apps present sensitive data (e.g. banking apps) or copyright-protected content (e.g. video streaming apps). App developers can opt to use the Android platform’s FLAG_SECURE setting to prevent the screen from being captured, recorded, or shown on “non-secure displays” such as the recent apps view. There are a couple of ways to disable this flag:

Smali Patcher module for Magisk. Magisk is officially supported on Android versions 4.2⁠–⁠11.0.
DisableFlagSecure module for Xposed. Xposed is officially supported on Android versions 5.0⁠–⁠8.1.

The Smali Patcher method requires you to connect your phone to a PC and run a Windows program to actually generate the module. Note that Magisk on Android 4.2 does not support MagiskHide.
The Xposed method has a big caveat: many banking apps use Google’s SafetyNet platform for checking device integrity, and prevent the application from being used on devices that have Xposed installed. This renders the module useless in SafetyNet-enabled apps, and there is currently no way to hide Xposed from SafetyNet.
Use these modules with caution, as they disable the flag system-wide.
Taking a screen capture of your phone on your PC
If you can’t or don’t want to disable FLAG_SECURE on your phone, you can also use scrcpy. It lets you mirror your phone’s display to your computer, and then you can just use a screenshot software of your choice on your computer to capture the phone’s screen. This requires you to turn on ADB debugging from your phone’s development settings, but beware that some apps restrict their functions when they detect that development settings are enabled. Scrcpy works on Android versions 5.0–11.0.