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The whole idea of virtual life and death is a big part of the emotional and financial side of vtubing. Shower people that depart with attention and money, they sometimes return to collect more money attention, the consumer believing "if I support her even more, I bet the won't leave me again". Both sides eventually grow tired and the cycle repeats
I don't think anyone is forced or encouraged to leave though, as it's usually the talent that profits the most from this cycle. Having the talent stay surely makes them more money than to bank on some short term merch, which for graduations is usually more expensive to produce as you are working with stricter deadlines before you can actually turn that into (less) profit.