Why are the 737 trim wheels located so close to the seats?
My guess is that the manual trim wheel handle (when not motor driven) to adjust it manually needs to be:


easily accessible from the seat.
generate a sufficient amount of torque by moment arm.


Which leaves this as one of the few spots where it could be conveniently placed. You can find the same thing in older aircraft cockpits including the L1011, B727 and B747-100/200. The 737 design remains very unchanged from when it first flew.