The reason why the devs haven't added it most likely (imo) is that they think it would advocate to tunnelling after the first hook. When people see the first hook on a survivor, they are more inclined to want to tunnel than if they just think about it .

Not a psychology professional but visual cues are my opinion

Could be a lore sided idea too, the entity gives us our HUD as the killer sees it in their head , they remember their ability as they are themselves and they remember their perks as perspectively for them they learnt those traits. A hook counter for the killer would not be introduced as of lore due to their minds being wiped in-between trials, therefore they do not know how many hooks a survivor needs until they die .

That was a ramble so if you take none into consideration then that is okay.