This has to do with the workload of Hogwarts teachers.
Subquestion: How many hours does a teacher of the core subjects (DADA, Transfig, Charms, Potions) teach a week? If this question has already been answered, a link would be very helpful!
In my estimate however, Umbridge hardly has any free time during the week to inspect other classes since she needs to teach her own. Sure, auditing Astronomy would be no problem since she doesn't teach anything at midnight, but what about the other core classes during regular class hours?
I seem to recall them having DADA at least twice a week up to OWLS, at least one of the lessons being a double lesson - making it 4h per week for each of the five grades, twice the amount since only half the students ever attend a lesson. All in all, she needs to teach 4h*5*2 = 40h for the lower grades. Then adding the 6th and 7th years who take DADA NEWT classes, that should add at least another 8h (4h per grade, the pupils not separated into different houses). So, when she needs to be in class for 48h per week -- are there even any regular class hours left when she doesn't need to teach?
Or does she simply go: 'Now, my students, read the next chapter QUIETLY while I go out and do some inspecting'?