Question

"explain why professional software is not just the programs that are developed for a customer"

Answer 1

To put it into simple terms, software is always a continuous process of coding and updating, thus, you will never code and be done. Professional software not only consists of programs that are developed for a client, but also the compilation of documentation, configuration of data, support websites, requirements, design manuals, and evolution to make these programs operational and complete. 

A professionally done software system consists of more than one program and configuration files used to set up these programs.