1) How do they know the % of unopened churches? Was there a poll done in the middle of a hurricane with 100% participation?
2) I know from friends with 1st hand accounts of flooding, that many church buildings we also flooded, making them unable to safely house displaced folks.
3) Opening up a building & making space is available takes volunteers, many of whom would be concerned with their own property, or unable to travel to the building because of flooding.
4) There are real and important liability concerns about opening a building as a shelter which must be considered.
5) Church buildings are usually not equipped for long-term evacuee housing.
All that said, I think congregations in areas that experience natural disasters like floods, tornadoes, ice storms, wild fires and hurricanes should be as prepared as possible to host people in an emergency situation. And that includes finding out about liabilities before hand and establishing a process for deciding if shelter need to be available.
That's why I'm proud of Hillside Christian Church - Kansas City and our dedication to serving the local homeless population through Family Promise of the Northland. We are pretty prepared to house up to about 15 or 20 folks if needed. And that is because we're regularly in the business of housing folks. We have air mattresses, bedding, sleeping areas, food supplies and showers.
I would not hesitate to open our building to those in need if we were able to help. But we are prepared for doing that, and many congregations may not be. I would urge all church-going folks to talk with their respective church councils or boards to see if there's a way they can be of good service to others during great times of need, for that is the call of Christ, and faith filled people must be responsible to that with the utmost humility.
As for now, do what you can - and that is pray for endurance and strength, & donate money to effective organizations who are already giving life-saving assistance.