Yes, developers generally just don't do it out of spite. Or it's a hideously complex process even once you've got access to the manuals and can figure out how to replicate the features.
I can offer you some detailed development facts about this situation. It's not that developers don't want to create complex systems for MSFS, it's that a major tool for doing so - THE major tool, actually - isn't available to us if we want to publish our work on Xbox.
I work for Milviz, but what I say here isn't their opinion, it's mine. And it's hardly an opinion at all, just facts.
In every version of FS I've ever worked with it was/is possible to simulate an aircraft at a basic level using default systems and RPN (XML) to interface to them. But for complex custom systems (PMDG, A2A, Milviz and others) it is necessary to use C++. Only a deep mature language like that has the power and heft to model the systems, say, of an authentic, complete 737 or even a 350i.
MSFS supports C++ in a system called WebAssembly (WASM) which provides a subset of the full capabilities of the language and its flight sim libraries. Enough for PMDG to do their DC-6 or 737 in MSFS for the PC.
Currently, the Xbox will not execute WASM code.
Period.
Therefore, devs are locked out of the Xbox if they code in C++ (WASM). This code will still work on the PC version of MSFS. Thus you have the DC-6 on the PC but not on Xbox. All Milviz aircraft use C++/WASM and therefore will run on the PC but not XBox.
This is the dividing line between PC and Xbox, not developer desires or intent. Everyone would like to have access to the Xbox market. But to do so, right now, your aircraft can't have any C++ custom-built systems.
We hope Asobo will implement WASM on the Xbox, but have no timeline of when, or if, that will happen.
Dutch