If you just get the engine running, sit there and let the student wobble all over the place and don't really teach him anything, then being an FI is dead easy.
If you actually want to prepare the aircraft correctly (fuel, oil, preflight), do the paperwork, brief him properly on the board, TEACH him the maneuver (as per the syllabus), debrief him thoroughly, do the paperwork, and prepare for the next student, all in a two hour slot (usually without a lunch break), AND earn a living, then being an instructor is a bloody hard job.
Anyone who says otherwise has obviously never been an instructor.