Why is a mediocre salesman more convincing than the best billboard?
Yes, the fact that you're quickly passing the billboard plays a part, but it's not the pivitol aspect.
A salesman, even a mediocre one, can tailor their message just for you. They can listen to the words you say, they can hear how you say them, see how you dress... and they can take all this info and use it to speak right to you.
But this isn't just for salespeople. Marketers can use this, too. And they have to if they want to keep up with their competitors.