Hello Gandalf,
I am not a member of the Wikimedia Maps team, but I assume that like many other organizations that render map tiles, they do not pre-render these tiles because that is a waste of disk space. Please refer to the following page for more information:
https://wiki.openstreetmap.org/wiki/Tile_disk_usage
In particular, pre-rendering all tiles would use approximately 54 TB of storage and majority of them are never viewed by the end user. So most map tile servers generally just render tiles as needed and then cache them.
Hope this helps.