Swapping items?

With the introduction of titanic hydra from ravenous hydra, one can swap between the two in game whenever they want at no cost. Why is this not the case with the last whisper items? They're two items that both build from the same component but have a different subcomponent, have similar but distinct uses, and cost the same. But you can't swap between the two at shop like with hydra. Was this just an oversight by Riot or is there a reason for it?

TL;DR Why can we swap hydras but not last whispers?