Why so many *arr services? Why not 1 service that works with all media types?

I am almost done building my first self hosted streambox through Docker. That's a total of 16 instances, each fulfilling 1 specific role.

As I'm new to the *arr world, could you please help me understand why it is standard to deploy multiple *arr services for each media type (ex: readarr1 for books + readarr2 for audiobooks) instead of using 1 that does multiple media types?

Thank you.