I don't think that's the reason. First, airlines don't own airports. So they don't benefit from your spending there. Second, they pay to airports directly (as i

I don't think that's the reason. First, airlines don't own airports. So they don't benefit from your spending there. Second, they pay to airports directly (as in gate/landing fees) and indirectly by having to hire staff at the airport. Flights with connection may cost less for various reasons. I'd think route optimization and price discrimination (direct is shorter and more desirable) would be the most popular ones.