Because There is no R sound in Sanskrit. Just like There is no Z sound in Sanskrit so you guys always sey Jour laga ker instead of Zour laga ker. As You replace Z sound with J sound.
We have separate symbols for Kh(Khuda), Q, Z,F, Gh(Ghazal) in Devanagari Script but use of it is optional and we can pronounce them in both ways in Hindi.
We also have separate symbols for your R but will write it as D in Roman script because its pronunciation resembles between R or D. And we also have a separate symbol to pronounce Ball as Ball and College as college and not as Baal or Caalej.