The reason that there are plenty of rocket first stages and basically no air-breathing first stages that go to orbit is that, essentially, this isn't true. OSC

The reason that there are plenty of rocket first stages and basically no air-breathing first stages that go to orbit is that, essentially, this isn't true. OSC Pegasus and Virgin LauncherOne are the only exceptions, and both are only good for very tiny payloads and accordingly have very limited flight numbers- compare the less than 50 total across both launchers to the 150 for just the already retired Delta II rocket (which entered service at basically the same time as Pegasus, is why I chose it). The only real use case that I can see for air launch is that they can be fully launch azimuth independent for launches from within US territory, so there can be some sort of responsive launch capability (read: military wants to launch something immediately). Other than that one use case, it's hard to see what air-breathing engines bring to the table for space-flight. Payload size is clearly a massive problem because airplanes are so volume limited. But there is another issue here: heating. In order for an air-breathing engine to provide useful thrust, you have to have enough air for it to breathe. That pretty much requires you to now have to deal with aerodynamic heating, which is a thing that most rockets try very very hard to ignore on launch. The most prominent air-breathing to orbit attempt going right now, Skylon, seems to be viewed generally pretty skeptically by most people in the space community, from what I can tell. And even then, the log-line is different: instead of separate jet engines and rocket engines, the idea is to have a rocket engine that can make a mediocre jet engine for some portion of the flight. Most people outside RE don't think this idea will work very well (I recall people who ran the numbers pointing out that HOTOL- the program that Skylon grew out of- would actually have significantly more payload if it had just rocket engines) but Bond and co. continue on, with very small budgets, the sort of budgets suitable for long-shot technology development. Thus, it appears that the people who control the purse strings generally agree that this has a low chance of working out, even if it is worth trying.