Because during "capture" you're merely copying what is on the linear hard drive (tape).
If you shot with separated channels/discreet channels, then that is what will be in the captured file.
thanks for reply- also would like to know how Ito split the stereo track into 2 mono tracks after bringing into Vegas? If I have a mic on the camera and a mic clipped on a person, I want it to sound equal on both speakers. hope that makes sense since I don't know some of the technical terms.
Duplicate the stereo track in Vegas. This can be done easily by Ctrl-dragging it down below where it currently is. That will create a new audio track with a copy of the audio. Right-mouse-button click on one of the audio events and choose Channels / left only. Do the same with the other but choose right only.