That would be a strange-looking little beast if they did!Related question: Why no high roof on 118 WB?
The marketing wizards have probably done their math on this, and established that such a configuration probably wouldn't sell enough to make the tooling worthwhile. Remember that the 118 has a shorter side door and thus has its own special, shorter, side door together with the tooling to make it. A 118/high would need its own special tooling because the door extends into the roof!
The bodyshells for these vans are designed to be as much mix-and-match as they can, with as many common parts as possible.
On a note related to yours ... you can't buy a 159" wheelbase low roof, either. The 136 is the only wheelbase available with both roof heights. There would probably be more of a market for a long-low than a short-high because of people who have to fit into places with height restrictions. But they don't do it if they don't figure that enough people will buy it that way.