For the vast majority of users, they would not want to have to enter yet ANOTHER login, face-id, touch-id, password prompt to get to their Apple applications such as Notes, mail, etc. And yes, you can leave your iPad, because your iPad will not let just anyone log into it.
Most of the time when I'm grabbing my iPhone or my iPad for an Apple Notes addition - it's because I need to do it fast, and I don't want to be bothered with a prompt that requires me to type in a password, even if Touch ID or Face ID.
From Apple's perspective, most users would not want this.
Those of us who are security minded and like to have 50 passwords, 2FA, etc, on our way to our data, ok, I can see where you're coming from. I loved having a massive 128 character+ password on my LastPass and salt it to the point that any change took a few seconds to save.
But it's not just sitting out there "you can't leave your iPad" and "it's impossible to protect your personal data" - this is not true and incorrect. I can't just take your iPad and get access to your personal data unless you unlock it for me first.
Do you lock every single door in your house?