Picked up ammo will he added to clips of guns equipped. It’s annoying to manually unequipped bullets as attachments then combine newly acquired rounds.
Health packs and flares should also fill out the max hold level of whatever you have in your equipped slots before going in your pack (inventory).
This has been requested a number of times already.
And I absolutely agree with you.
Let’s hope it gets changed in the (near) future. 
You only need to attach a type of bullets to a gun once. Then, for as long as that kind of ammo is in your inventory it will feed to the gun’s ammo pool automatically. It’s still not a perfect system, I tend to re-stack my ammo every once in a while to free up inventory space.
I think, the reason why they designed it this way is because the game shouldn’t decide what you put in your quickslots. Perhaps in the future this feature can be improved upon, I wouldn’t mind getting rid of attaching ammo to guns altogether.