@chai2,
Not only is it legal, you can get a license without an arm, or be completely armless.
The variables you gave wont ever change, some of them have been made safer though. Radio tuners in the steering wheel, child restraints,
drink cup holders.. Most models of GPS wont let you fiddle with them unless the car is stopped. Most have voice modes, you don't even need to look at the map. It's just like having a passenger giving directions.
Cell phones are a relatively new phenomenon. And because of that, the statistics and studies have been very clear.
How many conversations have you had on the phone and missed something someone else has said who was sitting right beside you, or an entire tv show?
We can blank out background noise, like radios or tvs, even people and often do. Our minds prioritize things, it doesn't make actions equal. How many videos have you seen of people walking into things while talking on their phone? Concentration is required.
There is a very good reason why people aren't given test to see if they can drive while using a phone, or drunk for that matter. Examiners don't need the added risk.