It's not that hard to maintain a random piece of paper, you do it with your ID, which presumably you do not keep on your phone. It is much harder to ensure an iPhone won't randomly be miscalibrated and shut off at 30%.
My ID is not a random piece of paper. A boarding pass is.
I have never ever in my life lost my ID or my phone. I have lost paper boarding passes on several occasions.
The failure modes of a phone don't really matter; if the phone fails for some reason (still has never happened to me), I can always ask a gate agent to print me a boarding pass.
I do it with my ID because I've conditioned myself to check for it everytime I'm going to move places, or rather, check for my wallet, which usually has my ID inside it, but has burned me on a couple of ocassions
Also, some people do keep their ID on their phones, either directly on the back of the case (I've seen it more on young women, maybe because lack of pockets) or with something like a magnetic wallet
Louisiana has digital ID, I don't use it but my wife does and it seems to work OK. I think you can have your phone locked with the ID up, which is the only way I'd consider using it, anyhow.
Its been a while since I've heard any issues with it. I prefer the physical card still.
Not it's not hard, but it is significantly harder, and why introduce unnecessary additional complexity into my life?
My ID is in my wallet. A sheet of paper doesn't fit in my wallet. So I have to put the folded paper in a pocket, where it might fall out when I take out my gloves, or I forget if I put it in my backpack, and in which pocket, etc.
A sheet of paper is extra. My phone isn't. And I sure as heck don't bring a phone with low charge to the airport... when I know I'm probably going to be using it for hours... and I've got my charger with me anyways in case I somehow did.
It's not hard, certainly. But I have lost paper boarding passes a few times. I've never lost my phone, though.
I just like not having to keep track of an extra thing. Travel is already often stressful; having one fewer thing to keep on me and avoid losing is nice.
It's easy to lose a random piece of paper. Whereas people are generally extremely aware of where their phone is at all times.