There's a *lot* of Roman structures in France to this day, but a lot of it is partial and fragmentary because it continued to be maintained, added onto, modified, and merged with later construction. In particular, the city walls in the core 'old city' of just about every significant French town include Roman-era sections that were integrated into larger medieval-era walls, and sometimes further walls all the way to the Napoleonic era.
The stuff that's in *ruins* is typically that which was already abandoned *during the Roman period*, and had decayed to the point of not being useful enough to maintain or restore by post-Roman civilization. For example, the great Roman bathhouse in Paris was no longer functional by the 3rd century, long before Rome actually ceased to govern proto-France.
