You know, back in the day, we used to have 3 express bus routes that ran from Downtown Baltimore and 2 suburban hotels straight to BWI Airport, no stops. They were coach-style buses with luggage areas in the bottom of the bus. They were actually very convenient, more expensive than the usual city bus, but a lot cheaper than a taxicab. Then they built the light rail to the airport, plus the Amtrak connection (which requires use of a shuttle bus) and the direct bus went away. The light rail takes about an hour from my stop, driving (or an express bus) would take about 35 minutes. Basically, when I go to the airport, I use a taxi or Uber/Lyft. The light rail isn't that practical.
I'm sure there are others, but the only direct express bus service in the US to an airport (designed for air travelers, not local airport workers) that I know of are the Flyaway buses at LAX. There's even one that connects LAX with LAUS. Unfortunately, the last time I was at LAX, they hadn't yet started the Flyaway Bus service to Long Beach, so I was stuck using a SuperShuttle, which is a converted 12-passenger van that stops at each rider's destination, thus making the trip much longer than it needs to be, but it was a nice tour of various LA suburbs that I never knew existed.
I'm sure there are others, but the only direct express bus service in the US to an airport (designed for air travelers, not local airport workers) that I know of are the Flyaway buses at LAX. There's even one that connects LAX with LAUS. Unfortunately, the last time I was at LAX, they hadn't yet started the Flyaway Bus service to Long Beach, so I was stuck using a SuperShuttle, which is a converted 12-passenger van that stops at each rider's destination, thus making the trip much longer than it needs to be, but it was a nice tour of various LA suburbs that I never knew existed.