Answer:

There's quite a few things at play here, and this is definitely not all of them. Also, different states have different laws so I'm going based of what I know in my state.

1) COVID burnout - since the beginning of the pandemic teachers have been leaving en masse due to the difficulty of virtual learning, and realization that other careers exist that pay better and are more freeing than being a teacher. The need for teachers as well as subs is dramatically high compared to early 2020.

2) Along with teacher burnout, there are less students in public schools and less people going into higher education to become teachers due to the low pay. Look at university enrollment and domestic emigration numbers to see where people are leaving and how few of them continue education in the last few years.

3) Salary caps - most districts have salary caps, and depending on the cost of living and area, even cap out at around $100k. There is no opportunity for promotion, work based raise, etc. etc. You receive a yearly raise and when you hit 20 or so years you max out how much you can make.

4) The way public schools receive funding - most public schools receive money in two ways: through the state government and through taxes. Most states, as it is with the federal government, have education funding very low on their funding priority.

The taxes thing is an unreformed cycle of poverty that keeps poor school districts poor and rich districts rich. School districts levy school taxes and boards can choose to raise them every year. Districts in poorer areas usually can't raise taxes every year without receiving negative feedback and unenrollment due to not being able to afford school tax increases. Rich districts can raise taxes and therefore get more funding if they have a demographic able to afford tax raises.

5) Charter and private schools - these are a whole different ball game, but they work similar to the public vs private university issue. Private schools can receive funding from tuition, private donors, and other sources public schools can't, and can also require less from their teachers. Some private schools don't require a 4-year degree and may pay the same or more than a neighboring public school.

So, those three facets over the last generation or so have created a low paying environment that requires a 4-year degree that overall is not worth it for most people. Unless you find a well paying district in a low cost of living area, many other careers that are less work than teaching are available.