Monolithic systems are faster to design and implement. Systems with decoupled components require more time to design, implement and iterate. A lot more time.
This doesn't just apply to kernels. It applies to anything in software; writing a huge monolith of intertwined code is always going to be faster than writing separate components with clear API boundaries and responsibilities.
Of course, monolithic software ends up being less safe, less reliable, and often messier to maintain. But decoupled design (or micro-kernels) can take SO MUCH longer to develop and implement, that by the time it's close to being ready, the monolithic implementation has become ubiquitous.
This doesn't just apply to kernels. It applies to anything in software; writing a huge monolith of intertwined code is always going to be faster than writing separate components with clear API boundaries and responsibilities.
Of course, monolithic software ends up being less safe, less reliable, and often messier to maintain. But decoupled design (or micro-kernels) can take SO MUCH longer to develop and implement, that by the time it's close to being ready, the monolithic implementation has become ubiquitous.