Imagine perfectly lining up 4 or more barrel linings with head counterbores, a real pain to manufacture... In a 2cv, you only have an individual head to line up on each side.
And separate heads don't seem to be the answer (look up VM engines and you'll know why).
Russ, what engine swap did you do to end up with 3 cylinders in your Seicento? Unless you're just not counting the one trying to become a steam engine

Its not the step thats important its the quality of the machining and that its only one pot it has to mate
Tink of a normal 4pot block much easier to press in 4 liners then skim the whole thing in a single pass than try to set and shim up 4liners (renault had that in the past)

As a useless bit of information I think may be correct... Bugatti back in the day, supposedly milled his engines so flat he didn't use gaskets.