The CDC knew this at the time. The "flatten the curve" message was "slow things down enough until we know more and can avoid our hospitals from being overwhelmed and more people dying."
The idea that the virus ever could have been stopped if we'd just all cooperated harder was a retcon invented later by people who wanted to criticize other people for not caring as much as they did. The actual experts always said the best we could do was spread it out.
This would be expensive, but as expensive as what we did was? Surely not! So, other regions providing funding and resources to the regions taking on the burden would be a strictly rational move.
You might say "oh, but people didn't know about the spread!"… but that's a ridiculous claim. The Less Wrong crowd tracked it in near-real-time from open source intelligence, and governments had access to more intelligence than that. The number of governments giving nonsensical advice, like "masks don't work because the respiratory disease is not spread via aerosols", and "replace your soap with dilute alcohol", lampshades a broad coordination problem. (We're not much past the "sweet-smelling herbs will protect from the plague" advice of yore, it seems.)
The things we needed to do were done – but for ridiculous political reasons, nearly everyone waited until after the disease had reached their regions to close their borders: internationally and intranationally, at every level! (The algorithm in Pandemic II's easy mode was more sensible than that.) So much of that effort, that psychological torment, was wasted. Even if the whole world had taken Australia's approach, we still would've brought the disease to manageable levels within a year. But there wasn't the political will… and so it goes. I think we're less prepared for the next novel disease outbreak, now.
It can be done. It just requires leadership, discipline, and the willingness to take strict, decisive, politically unpopular measures against violators and spreaders of misinformation. As Schwarzenegger said, when there's a pandemic on, screw your freedoms.
Forcing business owners to allow people of all races into your business was both unpopular and cited as an example of authoritarianism.
An American example: MLK never had popular support during his life. His approval rating around the time of his assassination was in the 30s or so. It would not be unfair to say that in the places that mattered most, he was wildly unpopular.
Doing the right thing is frequently unpopular at the time that you do it. There is a balance, but if you give everyone agency, you have to figure out how to keep the assholes from using their agency to infringe on another's agency.