1. you cannot not use them - the init binary doesn't gracefully degrade in the absence of the services provided by these APIs
2. the APIs are very complex and convoluted, with deep state maintained throughout the message exchanges; so complex it's effectively impossible to redo them without basically implementing everything exactly the same way
the fact that some binaries communicate through APIs doesn't mean the system is modular, if the "modules" of the system cannot be used independently for other purposes
First of all, most of these systems are totally voluntary. You can simply not use them. And if you use them, you can replace them with other components that speak the same API if you want.
The binaries talk to each other with a message passing API and that is actually the proper way to make a replaceable component. This has been done quite a few times before for many different reasons.