I still have a first gen (!) iPad that still lasts for weeks on a single charge when locked. It is useless now, because there is no software support, but not because of the battery.
While on they are constantly listening for a “find my” signal so it’s easy to locate. For the overwhelming majority of people it’s a good tradeoff.
"actually shutdown long press + UI" process"
This plus if you plug it in after to charge it it turns back on
Anyway, the root of their issue is other people unplugging it, which is a bigger issue than just the iPad. Still if you turn it off before pugging it in the iPad would have ~full charge if someone unplugged it. They hold charge for months on store shelves.
Something wrong with your iPad then. All three of mine would easily hold a charge for more than 2 days even when turned on but unused (so asleep).
I doubt the OP is making stuff up, but all my iPads (including an original model) simply don't show any significant battery loss when left unused.
Why didn't you try powering it off when done?