that can just be used anytime middle earth wanted them. Plus why do you think Sauron was sending out flying creatures with the Nazgul? He probably anticipated them using eagles and could’ve intercepted them. Frodo took it because he would be small and unnoticeable to most.
The eagles generally only help when things get desperate. In the Hobbit they came because the party was basically trapped. In LotR they came because there was need to try and save the hobbits lives when the mountain was crumbling. Those are the explanations I’ve heard.