That's because they separated. It didn't go as planned. If they would enter Mordor as a group they had better chance perhaps?

I think Mordor had some kind of anti-air defense (watch towers + crossbowmen)? Because if they didn't, it would be a persistent and major attack surface. And 40% of failure wasn't tolerable.

And after they separated, Frodo had no ways to summon eagles.

Moreover, maybe in the middle earth they see such occasions as a battle of strong wills, not firepower. A few heroes outwill a regular strike force. That's all what matters.