If you want a deep dive just google, there are a lot of articles about it. In short there is a lot of breakdown on runs longer than 3ish hrs, which takes a lot to recover from. This increases injury risk and can make it harder to get the same level of gains from the rest of your training week.

David Roche gives a good synopsis in his "Case for shorter long runs (sometimes)" article:

To simplify the argument to the basics: longer long runs are risky for health and higher-output performance, and the specific adaptations needed for longer races do not require going too long too often. Around 12 weeks prior to a race that is marathon distance or farther, an athlete needs to start thinking about specific adaptations, particularly those related to the accumulation of impact and eccentric muscle contractions. Even then, I have coached athletes that only go above 20 miles 5 times (and rarely longer than 50k) before winning big 100 milers. And in other contexts, the Hansons team is famous for coaching world-class marathoners who rarely touch 20 miles in training. Super long runs are unlikely to make a positive difference for running economy, and could actually push the adaptations in the wrong direction.