Because, as you said, it is about realism. You are flying French and German planes. They would be in Km and Metres.
Convieniently in the conversion of speed and altitude to metric 0 = 0

I see no harm in having a choice in the simplified gauges for people who would find it easier, but for me it isn't an issue.
"Yn y Nwyfre yn Hedfan"
"Hovering in the Heavens"
No. 10 RNAS / No. 210 RAF motto