I was just thinking about this today - coincidentally!
All Disney annual pass pricing (in fact, all pricing altogether) is really regressive. Annual Passes are rarely more than a few (separated) days' admission. But Paris takes it to an extreme, where it can (if you do it the right way) be cheaper than a 3-day visit.
I think part of it, is that Disneyland Paris is very much a weekend-trip park, and the cheapest pass ("Francillien") blocks out a LOT of days, most/many of which are weekend days. But "Fantasy" is only marginally more expensive, and allows many weekends, so it isn't a complete explanation.
Maybe another part of it, is that it's very clearly NOT targeted at non-locals. It's only available on the French website, and all the documentation is only in French. I haven't seen many non-French people in the parks with the passes. But again - I'm not sure why that would impact the price.
A final thought is that additional costs (restaurants, hotels, etc) are several times more expensive than other resorts - so perhaps they hope to make their money that way instead? I suspect that is the most likely explanation, but I've got no evidence. And they're clearly not trying to drive hotel sales through the pass, since Extra Magic Hours is a benefit of being a Fantasy or Dream passholder.
So this is really just a long post to say - I've absolutely no idea!