Actually, there are microkernels running in a vast amount of mobile phones nowadays. They typically don't run on the main CPU, though, but on various controller chips.
Microkernels are practical -- they just have different trade-offs. Microkernels are preferable if (a) the whole Linux kernel would be too large, or (b) you consider security more important than performance. It's probably also easier to provide realtime services with Microkernels, but I don't know much about Linux' realtime API to know if Microkernels would be much better here.
Given that most phones doing more than just providing calling and texting are running on monolithic or hybrid kernels, I wonder where you get this statistic. In addition, the most important part of secure and reliable systems is not what distributed safeguards you build into them, it's the amount of attack surface. If the monolithic kernel + drivers is smaller than the microkernel, it's probably more likely that the monolithic kernel is more secure.
Yes, most smartphones actually have two CPUs - the one running the OS and doing web browsing and stuff, which runs some monolithic kernel, and the one driving the radio chip, which runs a microkernel. They don't have much to do with each other.
I think the main reason for the separation is the realtime constraints needed by radio codecs - if your web browsing slows down because you received an email, it's a minor annoyance, but if you stop sending radio packets because you received an email, you probably lose the connection or worse.
Many kernels can give the necessary hard real time guarantees today. Honestly, I think that baseband chips run their own operating systems because it's illegal to sell open software radios in a lot of countries, and a cellphone where you could tweak the code running the baseband chip would essentially be one.
The radio CPU can be optimized for its job, so it may have extra instructions for error detection, decryption, etc. It may also be on a separate bus with the antenna peripherals, freeing up a lot of bandwidth for the main CPU.
It should be possible to do all this on a single CPU (real-time is no problem - just use an RTOS), but it would be expensive and eat a lot of power.
The other reason they split up the baseband and the main CPU is for regulatory reasons - the baseband is the only part talking to the radio, so it's the only part which needs to be thoroughly tested for regulatory compliance. This then lets them upgrade other parts more frequently without having to go through as much testing and obtaining approvals.
I'd say that's like arguing that "My language is better than yours at doing 'Hello, world!.'" This isn't a hard problem and even poor solutions can solve easy problems.
How is more privileged code more secure, I'm not even going to try to reason with you. The OKL4 and QNX microkernels are used in millions of devices from smartphones to planes.
Microkernels are practical -- they just have different trade-offs. Microkernels are preferable if (a) the whole Linux kernel would be too large, or (b) you consider security more important than performance. It's probably also easier to provide realtime services with Microkernels, but I don't know much about Linux' realtime API to know if Microkernels would be much better here.