Because the problem is not in the numbers, is in the way the item work, and they haven't either understod that OR they have understood but they are too scared to do the change required (and it would be a legit concern, but at least they should make it clear)
What should 
be? A hp health buyer buster (either a tank or a bruiser which happen to buy a lot of health), no?
What does do right now? Work better on squishies than on hp buyers
It is a paradox for a in item which damage come mainly from %health damage, but is just the reality of the situation
A tank/bruiser get more damage from %health damage on passive, but at least has high resistence and given the low raw damage stats the item BoTRK contribute little to the rest of his overall damage
A squishy has almost no resistence so it is affected by both the damage on passive and by the offensive stats the item gives and in the end BoTRK is more effective against them than against tank
And it will always be this way for instrinsec reasons as long as the item works this way, whether the item is gutted or overpowered (and at 7% will still be one or the other depending on the rest of the game balance)
There is only ONE way to change this, and it is put % max health damage alongside % current health damage, because it is the only way on which an item that work that way can affect hp buyers more than squishies
But I also see the concern on doing that, obviously.
Put % max health damage on an item everyone can buy is a big potential risk (unless the number is very low but then it would be ineffective), and would also show flaws in their design philosophy, as they are keep increasing the usage of %max hp damage on kits, even though gated by the famours 3-hit passives or things like those and would force them to put big amount of work to redesing the power balance of various champs that have %max hp damage as part of their power to avoid completely unbalanced situations with champ abusing the synergies with that and other items (es. pick/ban Vayne)
So, they are in a cul-de-sac (a dead end), they either have an item that is inherently unbalanced, and at that point better to make it clear if it is gutted or overopwered and intentionaly not go for a middle ground and make their balance levers easier to use for them, or make the change required that carry in itself a huge risk
And they should better clarify this thing once and for all, because the situation is, well, non sustainable.
They don't want to use %max hp on such a similar item? OK, but then let's make it clear to costantly work and gie special attentionon the champs that should use those items
They want to make the item effective for what is meant to be? Run the risk and put %max hp on it so you can stop with this actual situation, even if it means they got to do a massive amount of work on gameplay updates
will most likely see a lot more play due to assassins as well
does not have double attack speed anymore. And at the time they nerfed the damage or in patches since then they also increased the total cost and the cost of 
.
and 
which in combination with the bork of past seasons would be broken as fuck.
and 