Some vehicles/engines have a bleeder for getting air out of the system, normally you see these on the thermostat housing to get a trapped air pocket out so the thermostat will work and open before the engine itself overheats.
That's because air doesn't transmit heat as easily as water/ a liquid will so there is a delay of the themostat opening, meaning the engine will already be too hot by the time the thermostat itself will open AND the air will still be there later in many cases to do it all over again, leading to things like head gasket failure and so on.
In this case it's not a question of it being at the highest point, more like a place where air can easily be trapped and it simply cannot get out without a little help.
Another thing I've done before is while filling the radiator I'll squeeze the upper radiator hose repeatedly, inducing a sort of "Sloshing" effect in the system that helps get air out as I fill it. This can also be done after the system is filled and the engine is running, helping to make air move along to a place it can find to escape from the cooling system.
If you have a bad spot somewhere on the suction/intake side of the water pump, it might be drawing air in the system too, that possibly including a loosely clamped hose end on that part of the system.
Weak/bad radiator caps can cause a loss of fluid as well along with one that has a bad O-ring/gasket letting coolant escape while the system is pressurized.