This was the original business model for XCOR, i.e. build and sell rocket engines to other companies. In other words, they wanted to be to NewSpace what Continental and Lycoming are to general aviation. Unfortunately for them, it seemed that just about every NewSpace group (with the notable exception of Scaled Composites) wanted to develop their own rocket engines. Masten and Armadillo are specific examples of companies that could have benefited from not having to spend money developing their own engines. Ironically, Armadillo is now XCOR's competitor as both have sold engines to the Rocket Racing League.
Granted, that when NewSpace first started out back in the late '90s, there weren't many options for reusable high performance rocket engines. So it could be easily said that most NewSpace companies had no choice but to develop their own engines in-house. However, there really isn't much of an excuse anymore.
I've talked before about NewSpace v1.0 (Rotary Rocket, Pioneer Rocketplane, Kistler, Kelly Space & Technology, ect.) and NewSpace v2.0 (Scaled Composites, SpaceX, XCOR, Armadillo, Bigelow, ect.). I will say that we will know when NewSpace v3.0 begins when we see a more clear segregation in the industry between propulsion companies and airframe companies like we see in similar industries.