Apple products don't necessarily have the best reputation of charge capacity, but to Apple's credit, they get away from it by having a very efficient operating system to drain as little power as possible when using multiple apps simultaneously which is a good compromise. However, that doesn't necessarily explain why the charge time on the iPad specifically is so long.
The reasons for this is largely due to the iPad containing a far bigger battery than both the iPhone range as well as a bigger battery than the Macbook range as well with a huge 7,606 mAh battery unlike the current iPhone 12 at 2,775 mAh and the Macbook Air M1 with 4,379 mAh.
With that said, the current iPad still uses a Lightning connector cable and not USB C which also drastically increases the charge time as well. Even with the high powered wall adaptor the iPad still takes over 2 hours and 30 minutes for it to fully charge up at 20W.
Take a look at the video below by the YouTube channel: Apple Explained on Why The iPad Charges So Slow...