Farbwerte im Histogramm Lightroom classic

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You can also change Photoshop's units to [0..1] (comparable to percentages):

The reason is that in Lightroom Develop, the image has not yet been encoded into a standard color space. That's what would define numerical values. Numbers are color space specific.

It is in a color space (linear ProPhoto), but that's a non-standard color space that does not correspond to any scenario outside Lightroom. The histogram and numerical readout uses a special variety of that, where the linear tone curve is remapped into the standard sRGB tone curve - but the primaries are still ProPhoto (the latter is the one known as "Melissa RGB", presumably after the engineer who came up with it). So again, non-standard and not relevant outside LrC. 

In Camera Raw, the color space is defined in the workflow options. You could argue that the same applies to LrC when in soft proof, so it could show numerical values there. I don't know why it doesn't, probably for internal consistency.

It was a design decision made back when Lr was first released. Many, like you, have remarked that decimal values such as those used in Camera Raw would be bettter, but alas the idea has never been taken up by Adobe.

Note that there is an existing feature request along the lines you suggest. I expect this thread will be merged with that one at some point.