Why do airplanes need to fly so high?

[ChrisG2010]

I get they have to fly so high to make sure to clear all mountains and buildings and such, but why must they cruise at 33,000+ feet?

 

[Friendly Survival]

I get they have to fly so high to make sure to clear all mountains and buildings and such, but why must they cruise at 33,000+ feet?

I believe they have the best performance to fuel ratio at that height. I also seen that they will do odd for one way and even for other to maintain such.

 

[jyy]

Actually, that height is amazing, considering Mount Everest doesn't nearly reach that level, and people cannot live there. There isn't enough air pressure to bring in oxygen.

I believe they have the best performance to fuel ratio at that height. I also seen that they will do odd for one way and even for other to maintain such.

Would this only be for the big ones? How about or smaller ones going from LA to Seattle?