I don't know why it's the case, but it is very frustrating to me. I have 3 kids and 3 iPads, not assigned per kid, just a pool they all and my wife use. Current implementation is a common "family" account logged in as a "child" on all 3 iPads with app purchase approval going to me and my wife (we both have iPhones).
This "works" just kind of OK. But it would work MUCH MUCH better if my wife could have her iCloud account sign in from the unlock screen when she wants to use it and have the kids sign in from the unlock screen on the shared kids account. The way it is now my wife ends up signing into various services she wants to use on random iPads, which isn't really ideal.
Apple's solution to my family's problem would be to buy my wife her own iPad. But we don't have a shortage of iPads, there's almost never a time when someone doesn't have an iPad available to them when they need/want it. We have enough hardware, just the software doesn't provide a way to share that hardware in a nice way.
I think they have this for school iPads, with special management software, but not available on the consumer side.
I suspect they're pushing for you to buy a device per user. But even for a household without kids, I could see some utility in being able to pick up the nearest iPad and having your personal state on it. (I think ChromeOS does this, but I haven't used it.)
It would be huge for families, and it would also make parental controls way more of a breeze.
Does anyone know why this is the case? I was positive when they made iPadOS its own thing, we'd finally see this.