Unions in the US have to be more defensive, because they're constantly under attack. Unions and management are pitted against each other as enemies, and the only way for the union to protect its members is to make and enforce rules. Each of those rules is in place to protect against some past abuse, and in that hostile environment, they know that flexibility on the rules invites a return of that abuse.
Not that everything in Europe is sweetness and light with unions, but they are more widely accepted. That leaves more room for amicable rule-making and flexibility.
The anti-union forces in the US, of course, put all of the blame for that on the unions -- which is exactly the kind of hostility that makes it difficult for the unions to negotiate flexible, reasonable contracts.
Not that everything in Europe is sweetness and light with unions, but they are more widely accepted. That leaves more room for amicable rule-making and flexibility.
The anti-union forces in the US, of course, put all of the blame for that on the unions -- which is exactly the kind of hostility that makes it difficult for the unions to negotiate flexible, reasonable contracts.