AirBnB has limited control over the guest experience. For instance, delivering instant booking would be difficult, and it can't guarantee consistently fast WiFi. It could conceivably incentivize hosts to offer certain amenities, but not all hosts will offer them, they won't be implemented the same way and in many cases this could negatively impact price. Where a host doesn't live up to AirBnB's standards, AirBnB would often have a limited ability to rectify the situation promptly.
Cobbling together features like gym access through partnerships is not very appealing. Depending on the location of an AirBnB rental, it could in many cases be incredibly inconvenient to have to travel to a gym, and folks used to staying at higher-end hotels are not going to be thrilled with access to 24 Hour Fitness. Gyms like Equinox have no incentive to partner with AirBnB so that somebody renting a room for a few nights can use a facility that members pay a premium to keep somewhat sheltered from the riff raff.