are you totally ignoring the fact that it steals your opponent's movement speed and gives it to yourself so not only are you slowing them, your speeding yourself up making it easier to stick to your enemy.
also lets do some math
a champion with 2000 hp, with bloodhirster you'll have no difference it still a 70-100 ad item with 12-18% life steal
with BotRK, your doing 40 AD + 4% of the target's hp (80) and your regen 10% +40 hp, so at this point of health, BotRK is adding more damage than your BT, by 20 AD, and for your BT to gain more health you'd have to be doing at minimum 2000 - 500 damage per hit. (the difference between 2-8% to get 40 hp)
so with these calculations we can say that if the champion is currently at 2000 hp your doing more damage and gaining more life back with BotRK than you would with BT, though if you had crits, this would be different because a crit with BT could potentially do more damage and have more life steal since BotRK's procs are not affected by crits. if the target had more hp such as a bruiser or tank, you'll see higher numbers and thus makes BotRK even better.
now if the target was at 1000 hp, then here are the numbers for BotRK, 40 AD + 4% (40) and regen 10% + 20 hp which would require BT user to do at least 1000-250 damage per attack.
so even with a champion with about 1000 hp BotRK will do better than a fresh BT but will do worst than a fully stack BT.
question becomes whats more important, getting that target down to 1000hp or finishing them off when they are at 1000hp?
if your the initator trying to do as much damage to a full health target, BotRK is the item to choose, if your a clean up crew and only attacking injured targets that are below 1000hp then BT is a better choice for you. and dont forget I didnt even take the active into consideration into these equations, which is an additional source of damage and healing.