In Star wars that's because the journey that Luke Skywalker was going though was running in parallels with the first Irish King Nuada of the Tuatha de Danan, (in Welsh known as Llud) who lost his hand and his kingship due to the Irish requirement that Kings be physically flawless only to regain it with a hand made of silver that was magically attached to him.

https://www.wikipedia.org/wiki/Nuada_Airgetl%C3%A1m

If the bacta tank had healed Luke's hand then he wouldn't have gotten the cool robot hand which gave him a point of familiarity with his own cyborg father in their final confrontation.