I'd vote primarily operating costs, but the rest's in there somewhere.
I instruct part time - as a CRI I suppose I'm lucky to instruct within a school, and get paid FI(R) rates which are - hour for hour spent on site, about a quarter of my day job as a senior professional engineer. Freelancing I manage to get away with charging about half what my day job pays.
Which I do because I enjoy it massively, but no way could I afford to do it full time. If I went up to full FI I might get back up to what I freelance at, working in a school.
So we have, for all reasonable purposes, no career for FIs, save for pure love of the job.
That is flying of-course. The range of pay for a professional engineer probably runs from about £25k to £60k: around 4:1 from least to best paid qualified professional. For a full-time pilot it probably runs from about £16k to about £160k - 10:1. Both probably run up around £100k in training debt.
The difference is that engineering employers are having to look hard to get the good graduates they need, whilst flying schools and airlines are beating the newly qualified bods away with sticks and the only real "sellers market" for pilot jobs is at the very top - training captain level, hence the high salaries there.
If the market could be modified in some way, so that there was real value to schools in having top-notch flying instructors (rather than just a suitably qualified and not too embarrassing body in a white short), then it might change. But I can't see that happening anytime soon.
G