Because it's effective. If you're advertising specs like "grams of sugar in your cola", someone else can swoop in and make a competing product that surpasses you in whatever concrete metrics you advertised. If you can associate your cola with "warm fuzzy childhood memories of Christmas", it's game over for other competitors. They not only have to match specs, but now also displace feelings. People identify as Coke people, or Pepsi people.

Also for some products, you literally can't advertise the features; the most precise mechanical watch in the world is completely dominated by a 30 cent happy meal Casio digital watch. That's why advertising for luxury watches is all centered around a "you made it [to graduation, to financial stability, to the upper class, to retirement]" theme.

Because we don't make rational decisions. We make emotional decisions. If you have 30 seconds to make an emotional connection with millions of strangers, you shoot for the heart.