Because the public keys are baked into the OS trust store. For the exact reason of not being able to get the keys from the internet if you don’t already have a root of trust.
The other issues (trust worthiness of CAs in countries that have the ability to compel a ca to issue a fake cert -Australia say), are intended to be mitigated by the CT logging that is now required by the major trust stores. Sure your Aussie CA might issue a fake certificate, but in doing so they ensure they get a global distrust...
The dream is definitely not trusting certs which haven't been written to a log. I think that the path is actually in sight too. The CAB forum seems relatively on board.
You can experience this dream today by simply installing Google's "Chrome" browser. If you prefer a different browser you probably don't have long to wait, Firefox and Safari have announced plans to check CT (Apple says in Calendar Year 2018 but I won't be astonished if that slips) and it's something Microsoft's browser team are contemplating - if you care about trust in the Web PKI you obviously shouldn't use Microsoft's products anyway, but if you do...
In order for CT to really work, we will need a better way to handle actually distrusting CAs.
I think that includes a way for a site to have multiple different certs at the same time, so their one CA isn't a single point of failure.
Without this, we will always be dragging our feet in dropping CA trust, because it will leave some perfectly valid sites shit out of luck.
The other issues (trust worthiness of CAs in countries that have the ability to compel a ca to issue a fake cert -Australia say), are intended to be mitigated by the CT logging that is now required by the major trust stores. Sure your Aussie CA might issue a fake certificate, but in doing so they ensure they get a global distrust...
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