When Mark and Jenny Mummert heard about the coronavirus pandemic, they cut short their vacation in Cabo San Lucas, flew home to Atlanta to pick up one daughter and their two dogs, and chartered a flight to Ulery’s Lakes—a gated community at the Moonlight Basin resort in Big Sky, Mont., where they bought 23 acres for $975,000 in 2014.

“We are holed up in our little guest house,” said Mr. Mummert, 50, CEO of Arco Design/Build, a construction company. They arrived in March. “We flew out here with no return flight. We are not...