It takes an insane amount of expertise and endurance testing to produce a safe, reliable jet engine. These companies have been doing so for 60+ years at this point. There’s a lot of knowledge and investment that has allowed them to produce these things at any kind of scale at this point.
I used to work at Pratt & Whitney, and trust me a LOT goes into this. We also worked with some of the Rolls-Royce guys. There’s a reason Boeing and others don’t even attempt it.
This was a century ago, when aircraft engines were radial ones with propellers, and weren't all that different or complex than truck or tank engines (outside of layout and materials for weight optimisation).
Modern jet engines are technological marvels of engineering with extreme complexity. And there are massive advancements like turbojets, turbofans, high bypass turbofans, geared turbofans, open fan every few decades.
Any engine manufacturer from the 1930s could and did manufacturer plane engines. Nowadays it's so complex and capital intensive there are 3.5-4 companies remaining with modern efficient designs.
I used to work at Pratt & Whitney, and trust me a LOT goes into this. We also worked with some of the Rolls-Royce guys. There’s a reason Boeing and others don’t even attempt it.