It’s a requirement that the airframe and engines are sold separately dating back to the original reason why United Airlines was named.
Which is not to say that commercial jets can take any old engine. Even something like this 767 that was split between GE and P&W have specific structures related to the original engine.
Which is not to say that commercial jets can take any old engine. Even something like this 767 that was split between GE and P&W have specific structures related to the original engine.