Airbnb's underperformance continues.On the surface, the investment case for Airbnb (ABNB 0.42%) seems excellent. The short-term home rental platform is disrupting the travel market, providing an alternative to traditional hotels, and the business model is easily scalable.It's not just asset-light, but asset-free, meaning the company owns none of the rooms at which its guests stay. In fact, Airbnb has more rooms than any single hotel chain, with more than 8 million active listings worldwide, and an unmatched ...