Why...
... do we call music notes "do", "ré", "mi" etc ?

The notation "do, ré, mi, fa, sol, la, si" was created in the 1th century by an italian monk and was inspired by latin religious chanting :

Ut queant laxis
Resonare fibris
Mira gestorum
Famuli tuorum,
Solve polluti
Labii reatum,
Sancte Ioannes

The "ut", difficult to sing, was remplaced by "do" in the 17th century