Because 10's if not the low 100's of thousands of them are dead and no sane minded Russian of fighting age, wants to. Just a thought🤷🏻‍♂️.

Could be several reasons, most likely :

-Operation and maintenance of weapons purchased from NK

-Intel sharing about NATO weapons/tactics

-Intel sharing about RU weapon/tactics

-Give some NK troops warfare experience

US/EU also have troops filling the same roles on the UA side, whether it’s "officially non-fighting engineers", or mercenaries.