You're right, they're not. However, NBC is owned by General Electric - a company that heavily funds the Democratic party. (Gee, imagine that.) ABC is owned by the Walt Disney Company - I'm sure poor Walt is turning over in his grave right now. He was heavily anti-Communist and against many of the policies the current administration is backing and ABC is reporting on as "good". CBS is owned by Sumner Redstone - a Democratic supporter.
Fox is owned by Rupert Murdoch - an Australian-born American who has many British holdings. He has (big shock) Conservative leanings. The real difference here though, is that the Fox news programs encourage people to investigate what is being reported. ABC, CBS and NBC do not do that.