If the Lake Ponchartrain causeway didn't fail, why can't they build levees that won't?

Watching the Modern Marvels program on the History Channel about the construction of the two causeways across Lake Ponchartrain. They can't reach bedrock, but they drove piles 70 feet down into the lake bed. The causeways held up to Katrina.

There was a program earlier about why levees fail. When they're overtopped, the resulting waterfall undermines the dry side and causes them to tip over. Why don't they build them with the same engineering they use for bridges? There's probably a perfectly good reason, but nobody has explained it.