A more interesting question is why people do long runs at all, or I should say, a regular, "every single week", long run that is considerably longer than a normal day as a "must do" part of their training schedule. Two things I wish I had accepted when I was younger and had more time (and was willing to devote it):
1)Overall weekly volume (achieved in doubles) is more important than a regular long run.
2)For some reason, 2 runs in a day, even if it nets you relatively little extra mileage, is better than one. 4 and 8 in a double is better than a single 10 and I am still not sure I can say exactly why.
I ran my best on 90 a week with frequent doubles but rarely a single run over 90-100 minutes.
At this point, I priviledge doing a workout on Sunday to a long run - better use of daylight (as a master, I try to do something hard every 4-5 days) and I only go long on a Sunday if it falls between workouts - otherwise I do no long run. My long runs have actually gotten faster at the same effort with less frequency.