Why processes cannot preempt interrupts?

I know when an interrupt occurs the process running is put on hold, and the Interrupt Service Routine is called. The current pointer is pointing to the process that was interrupted and I was told that when an interrupt occurs it is not linked to a specific process. So my question is why only another interrupt can preempt an existing interrupt routine?

Also, when a process(p2) preempts another process(p1), who is calling the schedule() method?