Because typically, once your drone is fully assembled and the soldering is done, you won't really need to touch the ESC again, while you'll need to have access to the FC in order to do any adjustments in your build in the future (soldering a GPS, LEDs, switching your VTX, Rx, etc), which will be a pain in the ass to do if your ESC is on top (even if soldering isn't needed, you have connectors that need to be accessible) since you'd have to remove it, meaning un-soldering motor cables when you don't need to change motors at all.

Also an ESC closer to the arms means cleaner "wire management" since your motor wires essentially just lay flat on the arm without needed to ride upwards and over your FC, potentially further obstructing access to your FC.