>>99990351NTA but I've dono'd several thousand dollars to a vtuber so let me explain. (I'm also hoping some vtubers see this and understand what makes donothon rewards we like)
First of all, some of us just have a lot of money to spare.
If the donothon reward is good, then you're basically paying for exclusive intimacy with your oshi. Something personal that's just between you and her. And, if you're a big fan, she likely remembers things about you and genuinely appreciates you (plus she wants to keep her whales around), so she'll send you a really amazing gift that makes you fall even deeper in love. Totally worth it.
Also, like
>>99990688 says, I'd get jealous if somebody else got all of that and I didn't. I've given a lot of money for rewards that I didn't even care about just because I didn't want anyone else to getting it.