Enhancing the capabilities of operating systems is a double edge sword.  You do get to build something from scratch with new bells and whistles. The downside is that it's up to the developers to play nice with other software on the system as well as make sure the software can't be used as a vehicle for potential bad actions. 

Another approach to enhancing the capabilities of the OS is to orchestrate the existing features of OS in a simpler, safer way. An example is the operating system firewall.   Yes, you could build your own firewall but then testing this against all the different operating systems out there would land on the buyer's shoulder. Wouldn't it be better to use what is supported and provided by the OS vendor? All their testing you get for free as well as all the millions of customers who actually use the features currently.  

There are many useful capabilities in each and every operating system that are  just dreadful to implement due to operational issues. Companies like Illumio are mitigating those operational issues and allowing customers to use many features of their operating system without ever exposing the systems to new vectors of attack.