Why does Careersome split reviews, probation, and PIPs into separate workflows instead of one review system?

Because they answer different questions. A review cycle asks how someone is doing generally, on a schedule. Probation asks whether a new hire should become a permanent employee, once, within a fixed window. A PIP asks whether someone can turn around a specific performance problem, on its own timeline. Forcing all three into one generic form would lose the outcome, timing, and access rules each one actually needs.