Kirsche's fans trend towards older single weebs with decent careers, as opposed to most vtubers' audiences being late teens to early 20s and consequently broke af. Same reason MechWarrior Online has a super high spend per player compared to other "freemium" games - the target audience actually has lots of disposable income by simple merit of being old enough to have marketable skills. Plus, I'd be willing to bet this is a collective effort from multiple superfans who pooled money to do something that just one income couldn't achieve.