I believe "origin" is both pretty vague and hard to establish (as exemplified by several discussions that annoyed Antoine so much), and also provides little information to the reader (most people would know Fiat is a marque of Italian origin, and even if not, Wikipedia is a better place for such info).

I believe the main "purpose" of IMCDB is helping identify specific vehicles appearing in the movie, hence the minutiae detail we add such as specific production/model year, version, trim level etc. Manufacturing location can also markedly influence the vehicle, with versions made in different countries sometimes differing very significantly (like the Fiat Idea, whose Brazilian version is almost an entirely different vehicle from the Italian one), but also sometimes by the fine detail we love so much.

Not to mention that manufacturing location is much easier to establish with certainty than some vaguely-defined "origin".