I think it boils down to three things-
1) Marathons are a spectacle. Roads are closed. Network television crews are on hand. Fans line the course. Live bands play. Everyone gets a trophy. People run it for the experience.
2) Marathons are popular. Good advertising and media coverage has made the marathon the most well known event in running. So, when somebody decides they want to run a race, the marathon is the race that stands out.
3) Completing a marathon is an accomplishment. Despite this forums condescension, completing a marathon, even a very slow marathon, IS an accomplishment. You can’t just roll out one day and stroll through a marathon like you can for a 5K. It takes training and determination. It’s also an achievable accomplishment. Completing a marathon takes work but winning a local 5K (where there are bound to be a few more serious runners in the field) takes even more work… and time… and probably talent. It’s also an accomplishment that people care about. All a non runner knows about a marathon is that it’s a really long road race that shuts down the city and makes the papers and evening news. It must be a big deal to run a marathon. So when you finish a marathon people are impressed that you completed this massive undertaking. The time isn’t important because a non runner has no idea what a ‘good’ time is…. Just like I have no idea what a good score in golf is. You finished and that’s all you need to tell them to receive their praise and admiration. And yes, we all want praise and admiration. If you didn’t you would tell people on the internet about your 16 min 5K.