Apparently they are not really louder we just think they are louder because of the way they are made.
Generally, the producers of an ad have 30 seconds to get in a lot of information, so the words don’t have those natural pauses between them… they’re all jammed in together.
Then you add music, to give it some energy, and to fill in all those annoying noiseless gaps where the voice-over person takes a breath. Sometimes, the audio engineer will even “de-breath” the ad, deleting the sounds of breathing so more words can be fitted in.
