Because no prospects. Look the plant manager is the boss, and he is compensated for how efficiently he runs things. Now in a plant, output is more or less fixed since capacity is capped by machinery.

Hence for a steady state plant, where there arent huge efficiency leaps to be made, the only thing he can control control is input, or minimizing input.

So big picture is, if your big boss is basically compensated based on how cheaply he can keep things running, salary increments will naturally not be very exciting .