The problem is in the resolution.
A good webcam has a low resolution to transfer only as many pixels across the net as fit the screen.
A good camera has a high resolution to optimize the quality of the image printed on paper.
It's like a car. You can get a good car to transport yourself and three passengers from Los Angeles to San Fransisco, and you can get a good truck to transport 40 tons of goods the same way, but it won't be easy to find something that's suited for both.
You can do some things with software interpolation, but I'm afraid you won't be enthousiastic about the result. Have a look at http://www.techtastic.ca/reviews3/logitech-420.html for example.
You'll be able to find better ones in the cnet review section with http://reviews.cnet.com/4500-5_7-0.html?qt=camera+webcam , but it depends on your needs if you find the result good enough or satisfactory at most.
Have a good search.
I'm looking for a webcam that is also a good digital cam or vice versa. Any products recommended? I'm looking for a good balance between picture taking and webcam quality.