To an extent its because that where the vote block is. If no one stayed there longer than 10 years than year 12 pay would be year ten. Your assuming year 5 would be the current year 5 not moving year 12 to year 5. The senior steps are created as a way for the largest vote block to give themselves raises the company wouldn't give to the whole group or at least a larger raise. Most trade labor is equal work equal pay. Its not mathematically imposable to crush a 12 year scale into a 2 year scale.