I realise that I may be being cynical here, but this is possibly due to:

1) It takes longer to write code from scratch than to drop in a module, i.e. less billable hours!

but this is probably a minor consideration in comparison to:

2) If he turns his code into a module and supplies it to you, if you go somewhere else they will have access to this module to drop into other programs.

I do not know anything about the AMX programming world, but in CrestronLand modules are chunks of programming neatly packaged into a logic block designed to perform a specific task. You then only need to deal with what you send to, and receive back from the module without having to necessarily understand the processing logic that the original programmer built the module with.

Providing another programmer with large amounts of code to perform a task can make reverse engineering it for their own gain more difficult than if it as been 'modularised' (is this even a word!?).