Because developers generally don't know (or don't like) the outreach necessary to fundraise.

For example in numpy case - https://github.com/numpy/numpy.org/issues/9 That's a request in March 2017 to add a donation button to the website. Im not sure that 6 months back, if Numpy was legally structured to receive larger funding. I posted a similar comment (with many more replies) in the context of Octave and it's funding https://news.ycombinator.com/item?id=13604564

Tl;Dr Don't ask for donations - instead sell "gratitude-ware"

There are tons of people who WANT to support these projects, but you have to make it easy and accountable to do that. The best example that I usually give is Sidekiq.

@mperham is awesome that way "This is exactly why I disclosed my revenue: people won't know there's a successful path forward unless it's disclosed. I want more OSS developers to follow my lead and build a bright future for themselves based on their awesome software."

In fact, I believe there's a start-up to be done here. "Stripe Atlas for Open Source software"