Simply because started as 'cultivators'. And cultivators uses spirit stones (sounds so weeb)
We all started reading from Coiling Dragon, BTTH, ATG, MGA, ST, ISSTH or whatever 'classic' cultivation novel you could think of.
Before there is Qidian International (Webnovel), most 'oldies' came from Novelupdates, wuxia world and tons of fan translations sites... But since most projects are yoink to this side, here we are.
Some may wonder why many users complain because of this change. I don't know for others but here's mine.. identity and nostalgia