This is a balancing issue, apologies if this isn’t the correct place to post it. This was also tested on 0.H so things might have changed already.
The adjustable sling (and other similar mods) allow you to wear your gun like clothing, which comes with the huge caveat of torso encumbrance and a 500 wield time for the gun (because you have to take it off like clothing). Compare to the “load-bearing vest with sling” which incurs about as much encumbrance as something like light load-bearing + adjustable strap, except its wield time for the gun is ~30 depending on the model.
500 to 30 is an insane drop, and it basically makes the adjustable sling useless unless you never plan on wielding your gun with a zombie next to you, which you can also do with the vest with sling for zero downside.
The vest with sling allows me to ready my AR-15 faster than my rapid-draw holstered glock. Maybe the reason is realism? Like the vest with sling makes your gun be right there hanging from your shoulder and basically ready to fire? But at that point why can’t you do the same with the adjustable strap? Or maybe you can and I just never figured out, yall feel free to lecture me.
And btw I’m not trying to rail on the game’s balance, I actually love the intricate inventory/loadout management. This is just something that struck me as really oddly balanced.