There isn't really a Leftist mainstream news network. There are conservatives networks like FOX and then not so conservative networks like all the others.
There's bias all over for sure but the Left doesn't have people like Shapiro, who knowingly spread discord and hate through misinformation campaigns and bigotry disguised (rather poorly) as intellectualism.
The mainstream media is biased but again there's no Leftist equivalent to FOX, and the other mainstream news stations are biased in favour of the right by omission; they don't cover issues which could upset the establishment ie mass incarceration, ghettos, lobbyists, lasting effects of regime change overseas etc.
Where people believe there's a leftist bias in the mainstream media they're often referrin' to social issues like transgenderism and while I agree there is a bias there, it's not the same and policy or economic stuff.