A Question Regarding spawnicons & icon16

I’ve tried to create a spawn menu with a custom icon and place the icon into the icon16 located in materials, but Garry’s Mod refuses to mount the icon16 folder if it’s packed into a workshop addon, packed into an external vpk or placed in the addons folder. The only way the custom spawnmenu icon is working is when I have it placed in either garrysmod/materials/icon16 or either repacked in the garrysmod_dir. The same goes for spawnicons any explanation why Garry’s Mod refuses to mount these sorts of .png files outside of it’s own garrysmod folder?