Why do all fighters since the biplane era have low wings?


Ever heard of the McDonnell F-15?


Okay, when I said all I really meant to say most, or a lot. What I was
thinking about in particular were the WWII fighters. I can't think of any
propeller driven fighter with a high wing. Your comment about low wings
being easier for landing gear makes a lot of sense, many of those aircraft
stowed their gear in the wings, but I wonder if there were any other
reasons. The first jet fighters were also mostly low wing (f-80, f-86,
f-100, mig-15, mig-19, etc). Only the latest generation of fighters (f-15,
f-14, f-22, etc) are predominately high wing.



Why do most military transports (C-130, C-17, C-5) have high wings,


It allows the fuselage to be closer to the ground for easier
loading/unloading.


This makes sense. But going back to WWII again, why were the early
transport aircraft (like the C-47) low wing?



but all airliners have low wings?


BAe 146, ATR 72, Dornier 328, etc.


Again I meant most instead of all, and I was referring to the larger
airliners (200+ seats). Basically, if all other things were equal, why
wouldn't they have made the 747 high winged, since some 747s are used for
cargo?



Why are a lot of cold weather/high altitude planes high wing?


Aerodynamic superiority.


To clarify here, by high altitude I meant planes that are designed to take
off and land at high altitudes - the pilatus pc-6 for example.