With 5-6 hours of flight time I'm often lucky if it is only an 8-hour day. I almost never have access to a direct flight cross country, so tack on at least another hour plus taxiing, running to catch a connection, then waiting for the connection to depart. If you let airline algorithms/travel agents select cheapest available flights, the connection is quite possible entirely in the wrong direction, too, adding a bonus hour or two to flight times.

I've had companies crazy enough to ask if they could interview me on a cross-country flight day to save themselves a hotel night (after they were already planning to schedule the cheapest flights they could find), and I can only imagine those people have never traveled cross-country much.