Q: There are two reasons why. I like baseball. It is fun. And good exercise.

A: There are two reasons why I like baseball, it is fun and good exercise.

Is this correct?

No.

You have two separate sentences which could be separated by a semicolon.

There are two reasons why I like baseball; it is fun and good exercise.

Is that what you mean?

Yes. It's now correct.

Yes, your statement is correct. It explains that there are two reasons why you like baseball: because it is fun and because it provides good exercise.