Monolithic kernels are also generally more performant, compared to micro-kernels, it turns out. A bit counter-intuitive at first but, makes sense when you think about it.

Micro-kernels in general-purpose OSes suffer from a death of a thousand cuts due to context switching. Something that would be a single callback to the kernel in a monolith turns into a mess of calls bouncing between kernel and user space. When using something like an RTOS where hardware is not likely intended for general-purpose computing, this is not an issue but, when you start adding all of the complexity of user-installable applications that need storage, graphics, inputs, etc, the number of calls gets huge.