@road to nowhere: plenty of churches open their institutions during disaster. Having A roof and A toilet is better than none at all.
The primary problem, as others pointed out, is cost. It would cost them in maintenance and upkeep, someone would have to be responsible, they’d have to *gasp* cancel meetings and actually spend money and ask for donations that don’t end up at the headquarters.
The insurance issue is secondary, there is some risk but it is low and insurance companies wouldn’t turn down a claim, however the WTBTS is self-insured (aka not insured, they have to pay everything out of pocket). The primary problem is indeed their own costs.