Whenever the government doles out money, the incentives are to do it in return for political favours. To counter that, various processes and institutions enforce checks and balances and accountability. In practice, that takes the form of grant applications, tendering, and the like. That then attracts a bunch of grifters who want to effectively steal the government's money, so the grant process get longer and more complex, and things get more bureaucratic with heavy-handed checks and balances.

If a government anointed any given handful of OS organizations as preferred benefactors of donations, I'd expect grifters to infiltrate those organizations and parasitically siphon off the funds one way or another.

Incentives matter. Government incentives are to be popular, or attract the support of other people who are popular or influential. Being efficient or effective is only a small part of that. I don't know that there's a good solution to the incentive problem.