Why can't I install an .apk via TWRP

I'm trying to understand why I can't install an .apk via TWRP.

My WiFi does not work, and I don't have a file browser installed so I have no way to browse to the APK I've copied to the SDCard within the OS.

Why can't I simply use TWRP to install an .apk? / How can I install a file browser onto a rooted device when it has no network or wifi connection?

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