Why does the lost chapter mana regent disappear once built into something else?

The lost chapter items have 3 items it builds into that had 3 “subtypes” on them: catching, poking, and more mana. Why is the mana regen from the core item on 90% of mid lane champions lostonce built into something else? No other core item has that happen. The only one that even comes close are the hex tech items but they provide even more damage to make up for the damage once built. The one reason we build this item is for the mana sustain to keep up with our incredibly high costs that we have to use to CS from a safe distance or toss out to have impact on a fight. We don’t get to auto attack, tank, use our passive 3 hit damages or anything else. We require mana and we have no way to get it back in a reasonable way.