Why I can not use my digital camera as an webcam?

Because I find that any digital camera can also be connected
with a PC through an USB port, so, in my opinion, it should
be realized for a digital camera to be used as a Webcam by
developing a suitable software.

there are a few cameras that can do it. i know this because my friend's dad does it all the time. but as for the brand of camera he uses, i do not know...