A CFI doesn't even count as a 9-5, and I've done it. You still need to work nights (kind of required by FAR) and often weekends, when a lot of students are more available. You are not going to work and get paid for 5 - 8 hour days. To even get 8 hours of pay, you are probably putting in 10-12 hours per day. The only part that is right is you at least are home every night.
There's also the matter of steady pay. CFI is largely dependent on student schedules, weather, maintenance, etc. It doesn't take much to lose a week's pay.