No human being is by born mentally weak - they get nervous/sacred (and acts out of comfort zone), when they are out of their depth or unsecured or subject to constant criticism or has a history of regular failures. For PAK players - they are out of depth after playing cricket on UAE garbage, unsecured at highest level where they are constantly competing with better conditioned players than the rubbish PCB arranges as domestic cricket (& for the double standard in selection), they are constantly criticized by fans & media (for a reason as well) and they have lost several close games for failing at one/two critical moments.
A longer tournament like WC with 9 games will give them - 1. time to acclimatize & get used to the condition 2. no fear of selection as junior players are assured of their spot for the entire tournament, 3. being in a competitive environment for 6 weeks instead of QeAzam cricket, 4. a longer tournament gives them chance to come-back, one loss isn't the end of the world, which makes them relaxed.
I have seen PAK cricket more than most here - with a better preparation & selection, there is a SF spot to pick as low hanging fruit. After that, it's a matter of 2 great days.