The post processor creating the program for a CNC, which mills the PCB, uses the coordinates of the entities itself. Therefor the drawing must be in real world units.
Human readable dimensions would just disturb the process.
If you want to check the dimensions, you can, as @dellus said, add dimensions yourself in a working copy.
Or use the Measure tools in LibreCAD to measure points of interest.
Armin
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