When you think about it and what the process involves, the amount of modifications required in widening a car entails so much more work to so many more bits & pieces than 'merely' splicing in a section to extend (a-la limo conversions)
For example, there's no point in just widening the transmission tunnel itself. Instead the floor on
both sides of the tunnel would have to be widened. As would the diff, and the steering system, along with so many other structural, mechanical, body & interior componentry
In short its an enormous undertaking. Which is not to say unfeasably so, even aftermarket conversions, as evinced by this widebody Benz