Because these instances are often run as hobby projects and out of passion, not to be sustainable.
The Fediverse isnt corporate, we're a community and nothing will change that. We don't want to make money, we want to bring back what social media was supposed to be.
Additionally, it has become part of the culture of the Fediverse to regularly remind your peers to donate to your instance, as they're who are running this all for our benefit.