Perhaps by covering stories as a journalist, one moves farther left. It’s not like it _should_ be 50%, or that occupation and political orientation are independent
Additionally, America's "left" is very much the same as Europe or Canada's center/right. The argument could be made that there is probably a more even distribution if you don't use America's right-weighted scale.
I think it is true that reality has a liberal bias, I don't think we have too many quality journalists, although the editors might be the problem.