Why does my battery drain so fast

It’s hard to say going by the info you have given.

You can see if any of the tips below are helpful

If not, you can make an appointment at an apple store or authorized service provider and see what your options are or if the battery needs to be replaced

Batteries - Maximizing Performance - Apple

You can find out approximately what the battery health is with certain tools. The most notable one is coconutBattery for MacOS. It will give information like battery health, although the number is a "live" one pulled from the iPad in real time and not like the Battery Health reading available on current iPhones.

Have you tried a hard reset of your iPad by holding down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons?