Generally speaking, if youâ(TM)re running a for-profit business, selling things is better than not selling them. SpaceX is planning to make hundreds of engines a year, but theyâ(TM)re not launching yet, so they may have some unused capacity. They could make a couple billion dollars selling these to BO and ULA. Thatâ(TM)s not chump change, and it would allow the Starship program to earn back some of the development expense before itâ(TM)s flying regularly.
Aerojet Rocketdyne had access to the RD-180 and blueprints and manufacturing information for decades, and they havenâ(TM)t been able to build an equivalent engine. So itâ(TM)s probably harder than just getting an engine to look at. Not to mention the fact that BO often hires SpaceX engineers.