We did have the ability to talk to aircraft, but as Rabid Cougar notes, this was more common in the Pacific than in the West. But air superiority does not mean a plethora of planes at your beck and call.
While I'm not an expert on airpower history, I know some stuff, but I also have experience with the modern problem from the military today. Admittedly, we have fewer planes, but with higher bomb loads and accuracy. Still, some problems are unchanged.
I'd caution against thinking that air superiority means you have clouds of planes in the sky. You do have raids with 1,000 bombers, but you don't have that many groups of 10 or fewer bombers wandering around. In fact, you never do for the big birds, only A-26s or something. Fighters are different, but they don't blanket the battlefield.
First of all, only some of your planes can be assigned to ground targets. Lots of your fighters are hunting German airfields or escorting bombers. You will have a set of planned targets identified and targeted ahead of time. Sorties will be planned to hit them based on priorities. Then you'll have targets of opportunity, which is what you're talking about. A machine gun nest or small battery of artillery, is really low on the priority level.
It's important to note that the planes are still flying from England, which makes tactical, non-planned strikes very difficult. Your time over target is not too great. Better than the Germans in 1940, but still not all that long. So you can't simply wait around for the call. And there are still so many Germans around that you can easily soak up all of your strikes hitting trucks, tanks, small groups of men as you find them. Yes, that could include small guns, but it's more on an ad-hoc basis.
And at this stage in the war (close to the coast), it was probably more fruitful to call in Naval gunfire than an airstrike, because depending on availability, your planes could take 3, 4 or more hours to get there and you've got a timetable to keep up to. But even the Naval guns have lots of targets and unlike the Marines, you don't have the people trained to work with naval gunfire on a routine basis. And you have to be careful calling in naval gunfire against targets in close proximity to your troops.
I think for these reasons, and for doctrine, you use a ground attack. If you sit around calling in airstrikes against every impediment, then you're literally crawling across Normandy and open yourselves up to counterattack.