Its not effortless...and god forbid if your drop your smartphone and get it cracked. This headache is simply avoided with plain old paper reliability.
Then you print a boarding pass at the kiosk? But I've cracked a phone once in ten years, it's not really something I'm worried about.
And paper isn't reliable. For most people, you're much more likely to lose a random sheet of paper than for your phone to suddenly permanently stop working.
Those kiosks are gone from many airports. You're going to have to wait in line to get anything printed. And if you fly Ryanair it will cost you over 50 euro.
Some airlines in Europe check both the boarding pass and the passport/identity card at the boarding gate, to ensure people haven't swapped boarding passes within the airport. I think I see this on flights outside the EU, but I'm not sure.
It takes time to look up a passenger on a computer.
The plane will be delayed if they have to do that for everyone while boarding.
I'm just speculating but if they can simply do a name+DOB-based lookup at a counter, it seems to me like the boarding pass thing might just be for historic reasons where people would feel weird if they are suddenly tracked based on personal details instead of a ticket