Because even large trusts are still competing with each other. If the funding process within an organisation becomes too political, it will shrink. In general, a large corporate entity has good governance from the mere fact that it succeeded, it's good at allocating money to different projects and managing people. Governments are good at holding political power by playing through political alliance. Thus, I think they will tend to do a worse job in allocating grants. The process is likely to become more politicized than it would in a private organisation. There are of course exceptions, I am talking about general tendencies.
No matter who funds science, someone, even if it is the government, has to reason about the unknown unknowns when deciding what to fund and to which extent. The amount spent by the NSF is about $7B, which is a blip on the budget, and a blip in the amount contributed to private charities. The YMCA alone gets about $5B. The problem is that people just don't care that much about scientific research, and capitalism is good at producing what people want.
No matter who funds science, someone, even if it is the government, has to reason about the unknown unknowns when deciding what to fund and to which extent. The amount spent by the NSF is about $7B, which is a blip on the budget, and a blip in the amount contributed to private charities. The YMCA alone gets about $5B. The problem is that people just don't care that much about scientific research, and capitalism is good at producing what people want.