Simple, it is because the interests of the rank & file union members and the union are not the same.

All the unions want is peace and to keep the dues flowing. This keeps their representation cost down. During a strike both the employees and the unions income goes to zero but the unions representation cost go through the roof. These analyst and outside counsel aren't cheap. If you are the staff counsel at a union you get PAID THE SAME whether your guys are out walking the picket line fighting for better; with you having to work 60+ hours between talking to the various parties. Or you could convince your guys that they are only worth 90-95% of the other guys, it makes for much easier negotiations, and during the 5 years of the meat of your contract you can take extended lunches and still have time to cut out early for your sons soccer game while your guys are on the line flying worse schedules than ever, in crappier hotels than ever, trying not to get sick from their moldy crew meals.