It's not what gets hit the most (pumpkins, for sure Skeeter!) it's what does the most damage when it does get hit. You can get differential guards, but most people don't need them - it's hard to poke a hole in those things, unless you like jumping your Jeep in the middle of rock piles, or driving really fast in bad terrain.
Your oil pan, on the other hand, is not super easy to hit, but it is comparatively easy to poke a hole in, and really really bad when it happens. Especially if you don't notice.
Gas tanks are bad too, but already well protected. And I'd rather run out of gas than oil.
Tie rods are hard to protect - you mostly just try to get bigger ones.