On all lines the total amount of available bandwidth has to be split between upload and download. If you've got gigabits or even hundreds of megabits to play with then symmetric is great, but on slower connections is makes a world of sense to heavily favour download just because humans are better at consuming information than creating it. Consider how many hours of videos the average person watches per week versus how many they create in the same period. Same for photos, emails, articles, etc. There are people who have parity but they are in a pretty tiny minority.

That said, I hear there are people in the US getting 300Mb/s down and 10Mb/s up which is pretty fucking nuts.