I may be wrong about this but I believe when the union (ALPA for instance) and the airline management negotiate a pay raise for pilots it is up to the union to decide how that additional money is distributed across the pilot group.

I think it is because union leadership is comprised of basically senior captains at that airline who have the interests of themselves and other senior captains at heart then you will always end up with a "top heavy" payscale. Not to say this is absolutely universal, but it could be that your own union is betraying the junior pilots.

In a sense it is good to have something to look forward to as far as pay and QOL as you move up in seniority but the junior FOs should be active with the union to make sure everyone gets a piece of the action