Because then they would likely often start reloading at a bad timing out of the player's control, which would lead to frustration, and it would be abusable by someone who learns the reload timings (trigger a burst, walk away, wait x seconds for the reload to start and come back while the unit is reloading).
NO need for a reload animation, when a clip based unit has been out of combat for a specific time The belt is automatically replenished.
Reloading during combat tho should work as normal with animation.
I dont think The Joy of this game is based on ultra relastic ammobelt and reloading simulations 😀