Because Southwest has decided not to allow them to use Southwest's copyrighted data. Southwest, as a matter of corporate policy, wants everything to go through their own website. There are a few other airlines that do the same.

The absence of Southwest Airlines from major flight aggregators like Google Flights has been a long-standing point of discussion on Reddit, stemming from Southwest's "Direct-to-Consumer" business model. Historically, Southwest refused to share its data with third-party sites to avoid paying commission fees and to ensure customers saw their unique policies (like "two bags fly free") directly on their own website. However, a significant shift occurred in May 2024, when Southwest officially partnered with Google Flights. While you can now see Southwest fares in Google's search results, many Redditors still complain because you cannot "book on Google." Instead, the site redirects you to Southwest.com to finalize the transaction. Some users also report "ghost flights" where a price appears on Google but isn't available when they click through, which is usually a result of caching delays or the airline's "married segment" logic. Despite the partnership, Southwest remains fiercely protective of its booking data, preferring that customers use their Rapid Rewards account on the official site to earn points and manage reservations.