Teacher pay is based on years in the system because it is so difficult to measure individual teacher effectiveness, without incentivizing people to pay more attention to the "good" students and marginalize those who are difficult to teach.
It's politically difficult, not statistically difficult. Statistically VAM does a great job.
Paying attention to the "good" students vs the difficult to teach ones is not enforced by every objective measurement system, it's purely a function of how you compute the teacher's score. There are many choices:
# focus on the best, ignore the rest
# focus on the worst, ignore the rest
# Focus on the cheapest improvements possible
# independent of whether best or worst
# Somewhere in between mean and max
# 1 < K < infinity