@Mikey
Ferries must be keenly priced, relative to Chunnel if they're still drawing this many customers. I know nothing of actual prices.
Alternative ferry routes do exist… if you want to explore Normandy or Brittany. For points south, you want the Route du Sud, so you'd start with a far longer ferry crossing and then have to navigate your way south, rather than just follow the "no brainer", purpose-built route, from the Calais docks.
@TWR
Some of them must be using British ferry companies, so there's only the docking fees for them to make. If the Brits are on their way to Spain then it's only petrol, a few meals, possibly a one-night hotel stop. Not a lot of spending, all told.