Halo launched in 2001. Twenty-five years later, we've had advanced physics, ray tracing, destructible environments, and photorealistic graphics... yet enemies still don't show persistent bullet holes when you unload an Assault Rifle into them.
Imagine fighting the Flood and actually seeing rounds tear through infected flesh. Imagine Elite armor becoming scarred and riddled with impacts during a firefight.
It's a small detail, but details sell immersion. After all these years, it's one feature Halo still hasn't embraced.