Also, sometimes when you're waiting at the doctor's office you're waiting for important news that would trigger anxiety in the healthiest among us, and distracting yourself is a great strategy in those moments.
I also prefer to read books in these situations, or face with the thing which makes me uncomfortable. I don't like to have a puddle of thoughts which fester and attack at the most inconvenient times, making my life harder than it should be.
Not everyone is scrolling through endless feeds, either - I'm more likely to play a game. There are more than two things you can do with a phone.
The main issue with waiting is that I'm just waiting. I don't like waiting and don't don't see an issue with doing something so the time passes more quickly.
If you don't like something and have the tools to make it a little better, why would you not use them? Especially if it makes you a bit less irritated if they are running late - it makes things better for everyone.
If you need to suppress your thoughts by using a tool while being alone and not doing something (to be pleasant), maybe you should do something about these thoughts rather than suppress them?