The operating system itself provides efficient management of system resources, such as memory management to manage effective memory, process scheduling to manage CPU resources, and so on. If you don't have much complexity in the first place and don't need to allocate the use of computer resources for the sake of a large number of applications, why use an operating system when a dead-end loop can solve the problem?
One thing is for sure, the more code you have, the less stable it is.
So, for example, some embedded devices for missile control and satellites don't use an operating system in order to get extremely high stability and real-time performance.