[panpanna]

And significantly more fuel efficient?

[ac29 → panpanna]

Aircraft manufacturers don't tend to make Jet Engines. The MRJ uses a Pratt & Whitney engine, for example [0]. Pratt & Whitney is a US company.

The engine isn't the only bit that affects fuel efficiency, but it is a big part.

[0] https://en.wikipedia.org/wiki/Mitsubishi_Regional_Jet

[coredog64 → ac29]

Manufacturers are actively prohibited from this activity by US antitrust law. The “United” in “United Airlines” was a reference to the joining of Boeing and Pratt&Whitney.

Although aircraft are designed for specific engines, it’s up to the customer to execute a separate purchase agreement for them.