2 problems with that:
The resolution of the screen is limited. If you find me a screen that can represent as much detail as a map can, be my guest.
because the screen can't represent very much, it hides the less important roads as you zoom out to reduce clutter.
Problem 2: GPS software is dynamic. The screen is changing all the time based on where you are and what you are doing. There is only so much the computer can comprehend at once and still effectively guide you with short response time.
Now, I don't know about you, but when I look at a paper map I don't look at all 10^5 roads on a map at once. I am interested in at the most 8 at a time. And I think gps nav software does that quite well.