The reason why we should make it a milestone:
It takes time and money to produce items via automatic engines, workers, and your own effort
It's annoying. Imagine being compelled to produce items when you are intended to save the progress to produce at the final moments. Some people sometimes want to skip the missions related to produced items also.
You can only produce a limited number of times per day. As a result, this milestone cannot be spammed like travelling.
You need companies to achieve this milestone fast. It's obvious why