In general I think you are probably right. Many developer tools are small projects that can be developed by a single person, and it seems they often struggle to grow into to a bigger company. (I've experienced this struggle myself. For me, the problem was not lack of money)

On the other hand, I think Jetbrains shows that it is absolutely possible to build a huge company that sells nothing but developer tools.

There are also a bunch of mid-size companies that sell developer tools. I'm pretty sure there's more than 5 people at Navicat, Hex-Rays, or Panic, but I'm not sure how big these companies actually are.