If the sink backs up, then the vent fills with water and is no longer a vent.
The vertical sections drain away the solids well, but the horizontal sections don't. Therefore, code wants the horizontal unwashed sections to be above the flood plain. It's the long term solutions to keeping the system up and running no matter what.
Imagine if Japan had placed their infrastructure above the flood plain. It just makes sense to prevent water problems in the future. If those sections of pipe are where the sludge and water can't collect, then they will last as long as the home.
There are some cases where inspectors will allow piping with waste fittings below the flood level.
Here is a nice link to Bert Polk's plumbing tips