Yea, I figured it out the hard way years ago. I, like most, initially assumed Google Flights contained all airlines. (which is different than the issue of them showing the airline and not the price). But they do not include all airlines. The ones they don't include are simply omitted and one would never know these airlines even exist.

I use Kayak Direct to find those airlines (https://www.kayak.com/direct) Just type the airport code into the box and the expected month of travel. Kayak's list is more exhaustive and at least shows all the airlines that fly into any airport. Sometimes I see one that is not on Google Flights. I then go directly to their website for booking. As I say, this happens mostly for International legs, not US based ones.