Because, to be honest, if the Vampire Knives with the Healing properties weren't added to the game, NOONE. Yes, NOONE would have thought about it, everyone would say that it would be OP.
And yes, it is kindof OP, as you stated with your maths up there, but the devs decided to put it on a chest that needs a rare drop, which, is based on luck
You might even get it on early hardmode or even never get it throughout your gameplay, in this case you would have to FARM for it.
And I mean, Melee doesn't seem to need alot of healing, there's heart statues, there's campfires, even heart lanterns and POTIONS! The Vampire knives don't even help at all with damage, They're weak! Every player just uses them for quickhealing, making it more of a tool than a weapon.
That's why, I think, that there shouldn't be more examples of this type of ability, yes , it's creative, but it breaks the game a bit too.