Writing software for a domain is itself a domain of expertise, with a similar learning curve. It isn't enough to know the programming language.

For example, it takes years to develop the knowledge and idioms required to effectively write high-performance systems code, which is separate from the language the code is written in. You can have decades of experience in a systems language and zero experience writing modern systems code in that language. Same with embedded code, supercomputing code, etc.

Writing software is only "not difficult" if you've already learned how to write it.