Why can't Wi-Fi be enabled in Android-x86 inside VirtualBox? If I enter the Wi-Fi settings of Android-x86 (currently v8.1-r1) inside VirtualBox (currently v6.0.4) and turn it on, why does it just immediately get turned off?

Some Android apps try to find devices inside the network so it's essential (e.g. if you want to use Chromecast and iRobot Home).
If you're running Android within a VM, I suspect the VM software is not allowing the Android VM to interact with your WiFi hardware directly, meaning that Android does not actually see that there is any hardware available for accessing WiFi. If you check your internet connection on the Android VM, does it show you're connected to the internet via ethernet? 

There may be a way in VirtualBox's settings to give the VM direct access to your WiFi chip, but I highly doubt that would work based on the nature of running Android via a VM on desktop or laptop hardware.