It's a simple thing, so let me break it down:
The private sector exists for one thing and one thing only: profit. Businesses don't exist to make the world a better place. They don't exist to make our lives easier. They don't exist to bridge the race gap or the cultural divide or build bridges to a better tomorrow. Businesses exist to make money. Anything positive or ameliorative that comes of their pursuit for profit is auxiliary and thus secondary.
The public sector exists for one thing and one thing only: the provision of goods and services by and for the government or its citizens. Governments don't exist to profit off of you and me. Their primary purpose is to keep this shit going, and by shit I mean you and me and the infrastructure that supports the entire society's long-term survival first and foremost.
Why am I bringing this up? Hold your fucking horses:
The government has a direct, vested interest in the survival of the population, including the businesses that dot its landscape, and of mankind and its resources. A government is the only institution with the authority or even capability to coordinate or deploy very large-scale, society-wide decisions and technologies that can address mass-scale social, socioeconomic, or climatical problems if or when they arise. Sudden zombie viruses, meteors, pandemics, multi-state terrorist threats, and many more in a long list of imagined or real, proximal or distal, existential or secular threats to humanity and its persons can only be tackled by the far-reaching bureaucracies so ardently maligned especially by those on the right. The machinery is slow and sometimes wasteful but it works in a way that a large smattering of private companies never could.
Apple and Google cannot solve the world's direst problems. This is not a diss on those companies; it is a reality borne from the fact that, despite the noble aspirations of Sergey Brin, or the world-class vision of Steve Jobs, or whatever, companies have far narrower sets of immediate and long-term interests than governments have. The same goes for impact: Amazon can donate $2.5 million dollars in support of gay rights legislation, but only the government can craft and enact it. Even individuals whose privately amassed wealth is used to fund world-beating charities like the Gates Foundation to the tune of $33.5 billion endowments tackling everything from malaria to poverty exert only small-level change in the grand scheme of things, because they and their efforts are part of a much larger machinery that is almost always coordinated and deployed by governments, for which we are the better given the narrow sets of interests and economic lines of sight corporations are beholden to.
Then there's the fact that the total revenues of even the richest multinational corporations in the world, of which the Fortune 100 comprise a bulk, add up to a paltry $7.5 trillion dollars. This is not cash-on-hand, it is merely their annual revenue, which is roughly half that of the United States's GDP. That means that even in some GOP wet fantasy of the private sector coming together to solve the world's problems in a more efficient way than the public sector, the corporations would fall short without the help of the government.
So here's the money shot you've been waiting for:
Every time I hear some idiot say things like, "Why is [NASA's] Hansen's answer always give more money to government? Let's rely on technology to solve the problem.", it makes me want to strangle myself with the rope equivalent of a government-subsidized Stafford Loan, because, really, goddamn it, what's so hard about this?
End of rant.