The fundamental issue here is trust. If someone isn’t making money from being an open source seed bank, then they’re much more tempted to rip and run with the seeds. People are less willing to give seeds to such a central bank when they’ve been burnt by scammers in the community in the past.
The community has lots of trust but it’s based on reputation and often mutual exchanges. You’re suggesting a setup that would function based on many one-sided exchanges central to one party unfortunately and that’s difficult to develop the trust for.