Their members are probably better served by smaller organizations, just look at the teamsters to see what happens when a union gets too big. They start to serve the corporate interest more than the members because that’s who’s ultimately paying their salaries.
Two of them represent manangement and supervisors. The others represent different work groups based on classification. It's pretty common in large corporations/industries. UPS has different Unions for their drivers/part time workers, pilots, and mechanics.