The airport gym exists. A startup called Roam opened one at BWI in 2017 with showers, Lululemon rentals, the works. They planned 20 locations within five years. Nine years later, it's still the only post-security gym in America. Travelers love the idea. Airports did the math and quietly killed it. Terminal retail clears over $1,000 per square foot, roughly double what a regional mall makes. A 1,175 square foot gym selling $40 day passes loses that fight to a Starbucks or a duty free shop…