Why processes don't have the ability to run in kernel mode?

OS use kernel mode (privilege mode) and user mode. It seems very reasonable for security reasons. Process cant make any command it wants, only the operation system can make those commands. On the other hand it take long time all the context switch. change between user to kernel mode and vice versa. The trap to the operation system take a long time.

I think why the operation system not give the ability to process to run in kernel mode to increase it's performance (this can be very big improve)?
In real time systems this works in the same way?

Thanks.

Solution

There are safety and stability reasons, which disallow user-space process to access kernel space functions directly.

Kernel code garantees, that no user-space process(until being executed with root priveleges) can break operating system. This is a vital property of modern OS. Also it is important, that development of user-space apps is much more simple, than kernel modules development.

In case when application needs more perfomance than available for use-space, it is possible to move its code(or part of it) into kernel space. E.g., network protocols and filesystems are implemented as kernel drivers mostly because of perfomance reasons.

Real time applications are more demanding to stability. They also use system calls.