Because champions with high AD ratios- Wukong, Fiora, most assassins, etc- could build heavy bruiser without sacrificing anything for damage unless the HP:AD ratio was so low as to be ineffective. They'd be able to build something like Black Cleaver, Deadman's, Spirit Visage, Death's Dance, Titanic Hydra, and boots, and they'd be supremely tanky on top of sacrificing minimal damage. Why? Because all that HP isn't restricted to just their auto-attacks anymore; no now it would apply to all of their spells and abilities with an AD ratio. Like Fiora's passive, Wukong's ult, etc etc.
Titanic Hydra is supposed to give juggernauts a means of proactive waveclear by way of the AA passive and reward them for building mostly health by giving them a means to deal damage when their spells are down. Notice how Garen, Darius, Illaoi, Mordekaiser, Nasus, and other juggernauts all have at least one skill that empowers their next auto-attack. It's not meant for most outright tanks because most outright tanks aren't supposed to be a damage threat; they're supposed to be a CC threat and frontline to try and bypass.
The problem is that right now Titanic Hydra's too strong as a damage item on its own without any HP stacked up. Made worse by the fact that the active is an on-hit effect, synergizing well with champions that frequently use auto-attacks, like Shaco. That's why I think it should have a bonus HP ratio instead of a total HP ratio. With a bonus HP ratio it would be an item you have to invest in using by building HP, which isn't a problem for its intended users such as juggernauts and divers, but makes it immensely less optimal on assassins and skirmishers that need large amounts of AD to work at their best. It would also probably stop Titanic from crowding out Ravenous.