Why don't unions ask for more?

I'm hoping someone with knowledge of collective agreements and unions can help me understand why union members would agree to 2 weeks vacation. Doesn't a union hold more power for negotiation?

This is what I'm reading:

More than 1 year of continuous employment -> 2 weeks

From what I can tell this is less than most regular employers (maybe food industry is like that though).

I'm looking at forming a collective agreement at my workplace but seeing this result is discouraging.