The reason is because any dressing added to your salad will draw water out of the cells of the leaves via osmosis. Considering that iceberg lettuce is 96% water, there's a lot of water to draw out.

Season to taste with salt and pepper. Pour the dressing over wedges of the lettuce and garnish with the bacon if desired. (Published 2002) ...

You’re going to get what seems like a lot of dressing from this wedge salad recipe, but all those crevices in the iceberg lettuce will use it up. The iceberg lettuce is sliced into rounds ...

French dressing (aka vinaigrette) will transform anything from mixed salad leaves to tomatoes ... prawns and serve with Little Gem lettuce leaves or shredded iceberg lettuce for a classic prawn ...

Iceberg has a good shelf-life and can be kept for up to seven days in the salad drawer of the refrigerator.

Next is the perennial, burger-topping iceberg ... lettuce? It's also called Boston or Bibb lettuce. But unlike real butter, this is crammed with folate, iron, and potassium. No Caesar salad ...

Use the whole lettuce heart and quarter ... An uber-simple salad, this is a spinoff of the classic iceberg wedge with blue cheese dressing. But instead of iceberg, you're using romaine hearts ...