Why do iDevices power on to charge?

The iDevice software is essential for safe and effective charging. Apple uses intelligent charging with the battery condition monitored and controlled by the device into which it is installed. The thing connected to the wall socket which you think of as "the charger" is now simply a power adaptor delivering Volts and Amps to the real charger which is part of your Apple device and is controlled by the device software for smart charging. The old days when you simply connected a dumb charging adaptor and left it for 12-14 hours are long gone.

It is likely your other smart devices are also awake and software working, but their design allows a minimal startup for charging only without fully activating the display. The thought of uncontrolled dumb charging with a Lithium battery is simply too scary to have it any other way.

This has always been a feature of iPad and iPhone. If the device is fully shut-down, it will automatically restart when connected to an external power source - such as a compatible Power Adapter, or computer via a USB connection.

Should you choose to do so, you can shut-down the device - and it will remain shut down while automatically charging the device battery.

Unless you don’t intend to use your iPad for many days, there is neither need nor technical benefit in fully shutting-down the iPad; while sleeping, the iPad can continue to perform background housekeeping - including responding to notifications and alerts etc - while consuming minimal battery power. A sleeping iPad can easily sustain a week or more without requiring recharge.

As described, your iPad or its battery may have a fault - or the battery is reaching the end of its useable lifespan.

If you continue to experience difficulties, you might be best advised to seek assistance directly from from Apple Support - and request a remote diagnostic test. You can contact the Support Team using the Support link at top-right or bottom-left of this page. Alternatively and perhaps easier, you can initiate and manage your support cases from Apple devices using the excellent Apple Support App. If not already installed, the App can be downloaded from the App Store:

https://apps.apple.com/gb/app/apple-support/id1130498044

You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad, Power Adapter and cable(s):

Genius Bar Reservation and Apple Support Options - Apple

My best guess is to enable something important that’s required.