Intrastate flights are pretty common in CA and TX, plus FL and of course Alaska and Hawaii; indeed, TX and CA have had intrastate carriers in recent memory, and I suspect that AS and at least one of the Hawaiian carriers began life doing intrastate flights only (IIRC AS still had some affiliated regional carriers that serve the state only). I would assume that there are still flights between PHL and PIT, DCA and some places in Virginia, SEA and some places in WA state, DTW and some places in MI (including some subsidized flights to the Upper Peninsula on tiny planes), etc. At least preCOVID-19, there was service (including some DL mainline) from MSP to Duluth and Rochester MN. I'm tempted to add STL and Kansas City, but I'm unsure whether the airport is in KS or MO (my guess is KS).