For North American carriers, it's culture, passenger preference, keep them calorically fed, and tendency to make food as least offensive as possible. Whether it's to cater to a general/Americanized palate or cater to minimize allergies or to avoid any exotic (or expensive) ingredients/dishes.

I know cost is a factor. Still, there are lots of tasty inexpensive dishes served by other carriers, such as AK Nasi Lemak or Chicken Rice or TG Red Curry. Just 2 weeks ago I had a great SQ Seafood Tom Yum Bee Hoon in Y. I just don't think UA would ever consider such options given their passenger demographic.