Barnacles tend to aggregate on pretty much anything - smooth or no. Talk to any boat owner.
The only reason they'd be barnacle free is if the paint is in some way ablative, and comes off in layers - which is how marine anti-barnacle paint works - it's very unlikely they'd lack them otherwise.
There is also paint that is poisonous to the barnacles- that's how anti-barnacle paint used to work in the marine industry. But having all of that poison (for example, lead was famously used back in the day) was determined to not be good for the our environmental health.
Since planes are in a different environment, it's possible that they use paint that is toxic to barnacles (but perhaps is otherwise unsuitable as marine bottom paint).
The only reason they'd be barnacle free is if the paint is in some way ablative, and comes off in layers - which is how marine anti-barnacle paint works - it's very unlikely they'd lack them otherwise.