Wifi

Hard to say. What happens when you try? The wi-fi selection in Control Panel will just disable the current network you are on until the next day. If you wish to turn off wi-fi permanently, you need to use the switch in Settings. Here is an explanation about the settings in Control Center when it comes to wi-fi and Bluetooth. Use Bluetooth and Wi-Fi in Control Center - Apple Support

Because it's not designed that way.

The Wi-fi switch in control panel is only meant to disconnect you from the currently connected Wi-Fi network, but does not disable Wi-Fi.

To disable Wi-Fi, you need to actually go into Settings->Wi-Fi and turn it off.

You can completely disable WiFi from iPad Settings.

From the Control Centre (accessed from top-right of the iPad screen) you can temporarily disable the WiFi radio. You can simply tap the WiFi icon in the Control Centre, or tap-and-hold and of the radio icons to open a further page of settings.

What is not possible? As I and the others explained, you cannot turn it off in Control Center. That is a temporary disabling of the current wi-fi connection. That is the way it is designed, and I provided you with a link to Apple's support document that provided an explanation to the behavior.

What happens when you attempt to tap on the wi-fi icon in Control Center? What troubleshooting have you tried? Have you tried just restarting the iPhone? Have you tried force restarting the iPhone? Force restart iPhone - Apple Support