Most bombers dispense ordnance from their undersides, ideally near the center of gravity; limiting the structural members needed in those parts simplifies things.
> About half of all propeller driven passenger planes have high wing.
These are designed not to require ground vehicles for loading and unloading. Propellor ground clearance is also an issue for the larger types.
> Unlike the An-124, the An-225 was not intended for tactical airlifting and is not designed for short-field operation > It still has high wing.
The An-225 carries cargoes of unusual bulk to unusual places. Being able to get these cargoes on and off the aircraft without some sort of lift has advantages.
Low-wing freighters, which are all variants of passenger types these days, almost always carry palletized cargo of standard dimensional units (See https://en.wikipedia.org/wiki/Unit_load_device) - this allows for standardized ground handling equipment for most loads.
> And U2 has high wing. It's probably the plane with most carefully calculated weight ever. > If it was inherently heavier, this would make no sense.
The U2 is not a high-wing aircraft, it is a mid-wing aircraft with its spar roughly running through its center of gravity. This is actually the most efficient and aerodynamically cleanest configuration, and many high-performance aircraft use this arrangement. The main problem with it for passenger or cargo use is that the wing structure ends up interfering with the middle of the fuselage volume near the center of gravity, which tends to be valuable space in a transport aircraft.