The other day I went to a baseball and the line was delayed because the guy trying to scan his ticket from his phone was having internet trouble (not the same as the problem in the article below).
Another annoying thing about digital tickets is the fact that you can't always very easily give one to a friend with whom you're attending an event. If they have to go to the bathroom, they might need a ticket to re-enter their section, and you can't give them your phone because it'll lock with 3-tiered authentication.
(Yes, I know that I work in tech, and yes, I know there are "solutions" to all these problems. It's still a valid opinion that digital tickets shouldn't be the only way. I would prefer to have physical ticket and then digital option as a backup in case my physical ticket gets damaged or lost. I don't want to install a stupid Alaska Airlines app on my phone.)