Unions are supposed to fight for each of their members equally. As such it is in their interest to eliminate performance based pay as much as possible. Increases in pay should be based on time in job (positively) and likelihood of losing job on time in job (negatively). If you have performance based pay someone who’s three times as good might get paid more, or long serving staff might be let go before the newly hired. That is not what the average union member wants, they want security and stability and they work to get it.

Nowhere is a perfect meritocracy but a union that did not actively work against meritocracy would not be working in the interest of its average member.

Unions operate according to the rules the union members agree to. There is no "one way" that unions function. A "programmer's union" would be structured according to whatever makes sense to the programmers.