In most parts of Europe teachers are public servants / state / government employeed. As such, they are bound to predefined salary ranges. Upside, they are employeed for life.

And no every union is about teachers, which are by defenition not comparable to white collar tech employees (coming back to the FB example).

And how would an individual teacher negotiate salary ?

How do you reliably measure teacher quality?

How do you reliably measure developer quality?

The same way you measure anybody else: performance reviews, which are usually executed around here by an administrator or 2 coming to observe a class every semester. Coupled with standardized tests as a secondary measurement, it's pretty easy to tell who's good at their job.

The problem is that developers and teachers are not assembly line workers, and any performance measurement metric you come up with will be optimized for with poor results.

Grade teachers on their students' test scores? Okay, now you have teachers spending inordinate amounts of time on overfitting the curriculum to the local standardized testing requirements and less time actually ensuring their students are educated.

Grade developers on their lines of code or number of features shipped? Be prepared for needlessly verbose code and buggy garbage shipped out to meet the metric.

Throw these objective criterion out the window in favor of a more judgment-based approach? Be prepared for lawsuits when the mistake is eventually made.