For one reason...There are already tons of public
LIDAR datasets with
point data down to millimeter.
I've even seen monuments and public art sculptures that have LIDAR
datasets down to millimeter precision using WGS84 base references.
1. For objects we store in wikidata, what exactly is known with this
precision? E.g., if you have https://www.wikidata.org/wiki/Q5577191
which part of it the coordinate specifies up to millimeter? That is a
big monument, lots of possibilities. No way to know.
2. For objects we store in wikidata, this is probably not stable - even
if we somehow defined that the point we know up to millimeter is the tip
of Schiller's nose in Q5577191 - that probably would change with weather
conditions, erosion, ground shifts, continental drift (on millimeter
scale, that becomes relevant), etc.
3. For any practical use - such as querying - having data for one statue
only with millimeter accuracy is useless, to really do some calculation
where it matters all statues (or at least all statues, say, in New York
- or at least most of them) should have such accuracy. That obviously
won't be the case.
For this reasons, I don't think millimeter accuracy would ever be
relevant for Wikidata.
Having it all queryable in Wikidata ? hmmm... not for
me, other data
catalogs and GIS systems handle that job.