1. If an individual sees drug advertisements with particular symptoms they might think that they have them, which would make their concerns sound more probable to doctors when they complain about it.
2. In the US medical malpractice suits are a big issue, so that fear often causes things that might be considered on the edges of unnecessary in most countries to be converted to "maybe, I don't want to get sued if something does happen". This is magnified by point 1.
Take more people thinking they are sick, multiplied by multiple conditions, multiplied by doctors effectively excessively afraid of false negatives in diagnosing, and you have one big public health concern and some big marketing bonuses.