Yes Frank is right. But you will see slang in dialog, of course, because dialog is written conversation. For example, hacking something together: On the way to the party, Ben said "I ain't going near Susan tonight". "How come?" asked Roger. "She is trouble, big trouble" said Ben, shaking his head.

'How come' is more spoken and less formal. In writing, especially any kind of formal writing, you would never use 'how come'.

why --- reason