Newbies don't like to shoot because they don't want to give their position away. Chances are you've been seen already anyway. Another reason newbies don't shoot is because they think they'll be hit by return fire. This doesn't always happen. Players miss and they don't always see where you're shooting from.
Newbies think the longer they stay in the game the better they look.
The final reason they don't shoot is money. Newbies usually don't have enough money to buy a case of paint every time they play. This is understandable, but tagging opponents is half the fun. Take your shots in moderation. If you use a pump shoot every 3 or 4 seconds.