Pick a random origin or destination airport: which domestic carriers are you most likely to find at the selected origin or destination airports?
I mean, Mesa is a domestic carrier, but you're not going to find them at a significant percentage of airports. Similar for Jet Blue and some others you listed.
The small airlines thing used to trip me out when I was flying nationally on a regular basis. Was often a case of "yeah, it was ticketed through <AIRLINE> and the plane bore their livery, but the flight was actually operated by <REGIONAL_AIRLINE>"
And, while I loved JetBlue, they rarely went to the airports I wanted to go to. Worse, they never left from my closest airport (<10mi from my house), meaning I'd have to catch a $75 cab to get to the airport they DID operate out of.