You are a great example of what non-TT faculty should be. Indeed about 1/2 of all adjuncts fall within this type, or professionals who teach specialized courses part time, or other such decently paid non-TT faculty who are pretty happy with their situation.
Unfortunately, as I have mentioned, around 1/2 of all adjuncts are those who universities hire to teach intro courses, and are hired per course, per term. They are underpaid and under supported. They provide a much less than ideal education since not only are they provided with very few resources (no space for office hours, for example), but have no ability to enforce standards. As it is, even in many “top” schools, few faculty even know who taught the intro courses a couple of years back, and there is no way to actually evaluate the level of mastery students have on intro topics.
They have become so ubiquitous that there are many highly regarded universities in which undergraduates will only start taking classes with a TT or tenured faculty when they are Juniors.
That covers a lot of it. The other part is that states have cut funding to public universities, but have, in many cases, frozen tuition. That means that the administration need more research money, so TT/Tenured faculty need to spend more time writing grant proposals. Funding for NSF and NIH is far lower than the research demand (if I told people how little Congress has allotted to fundamental research in AI you wouldn’t know whether to laugh or cry). SO competition for grant money is higher, meaning that faculty have to spend more time writing grants in order to get funded.
So, if faculty are busy writing grant proposals, that is less time for teaching. Faculty can usually use grant money to reduce teaching loads, but the university uses a lot of that money to supplement all other things except teaching. That means that very little is left for paying instructors.
In Humanities, it’s worse. State legislatures do not value humanities research, and consequently, neither do the boards of trustees. However, Humanities departments are supposed to teach many of the gen-ed courses, like all the English courses, History courses, etc. But the universities do not want to hire any new faculty in these fields, so they go cheap. Very cheap.
Academics in the field do not do the field any favors by continuing to churn out people with PhDs who will never get a TT job (because production of PhDs is tied to the ranking of the program).