The same reason why you won't find in-depth reviews of the 85mm f/1.2 S neither.
The argument that the buyer pool is mostly predetermined by things that are already known without reviews, ultimately applies to any lens, especially fixed focal lengths.
Both lenses started shipping this month, you'll definitive see a lot of reviews of both lenses in the next weeks.
Similar to the Z 85mm f/1.2 S there are almost only previews, first impressions or hands-on's of pre-production units, but no in-depth reviews.
Certainly the Z 26/2.8 does not generate the attention and clicks other Z-lenses like the Z 85mm f/1.2 S may generate, therefore we'll probably see less reviews, but the reason why there are mainly only previews, hands-on's, first impressions of pre-production units at this time, is primarily due to the fact that both have only been delivered for a few days.
The same reason why there are no in-depth reviews of the Z 85mm f/1.2 S, but maybe the Nikkor Z 85/1.2 is equally uninteresting and undignified for a review.
With lenses from the major manufacturers, Canon, Sony, Nikon, there are always heaps of reviews over time, of almost any lens, completely independent of the price level of the lens.
Total different story with small 3rd party lenses like Cosina/Voigtländer, where you can't find reviews of some new released lenses for months.
In the end, it is always the same procedure and will not turn out differently here either.
For the release of a new lens/new lenses there are plenty of pre-views of pre-production units from different sources/influencers.
After that it takes its time and reviews trickle in over time, since a serious in-depth review is much more time-consuming than a preview/video preview.
The release coverage, the additional buzz generated by aestetic video previews at great locations, like the Matt Irwin previews, are much more important today than the over time following, few elaborate in-depth reviews that really go into depth.
Especially since such in-depth reviews with many measurements and charts are only really interesting for a small part of the potential buyers.
Most are looking for positive purchase confirmation, and for that, less elaborate reviews, previews, hands-on's that don't go into minute detail are enough, which is why extremely in-depth reviews kinda rare these days.