This is something I've noticed as well. Pretty much EVERYONE thinks they are an "above average" driver.
Personally, I only have one friend who I think is an exceptional driver.
I sometimes think I'm a super good driver, but then I usually do something profoundly stupid so I get put back in my place.
The problem is that understanding vehicle dynamics isn't a straight forward thing, and applying that understanding to real world situations is even more complicated. And, sometimes it takes a few years behind the wheel for people to stop thinking they are invincible. You might be young, you might have great reactions, and you might be able to do j-turns in an empty snowwy parking lot, but how does that help you when somebody cuts you off because they forgot to shoulder check when merging right while you're approaching them with 20mph of overtake? Everybody makes mistakes on the road, nobody is a perfect driver, and there will always be situations that are out of your control.
Don't get me wrong, I'm a car guy and I like to dick around on the road as much as everyone else, but there is a time and a place for that sort of thing. I just don't think a merging lane in the rain is one of those places. 99.99% of the time, nothing bad will happen. Too many unforseen things could have gone wrong, though, and at 80 mph you don't have a lot of time to react. Plus, if you don't have a lot of experience and practice with vehicle handling at speed, you're liable to over-brake, over-correct, etc etc etc. Too many variable, not enough time or space to sort them out safely.
A small-ish anecdote that drove this home for me. I was in the fast lane doing 65. I needed to merge right to make an exit. There was a heavily overloaded truck towing a tandem trailer loaded with fishing gear, aluminum boat hulls, a 4x4, some BBQs, etc. I had space to pull in front of him, but said to myself "I'll wait a bit for a better opportunity, that guy looks like trouble." He had a lot of unsecured stuff in his back, and his driving seemed off to me. Traffic was a bit heavy, as it was nearing rush hour.
We crested a hill, and traffic had come to a stop up ahead. I hit the brakes, started to slow with traffic, needed to slow fairly quickly as the slowdown was a bit unexpected (but nothing serious). The truck, however, had to stomp on the brakes to stop his heavy load. And his front tires locked up. He had long treadwear hard compound rubbe, and he didn't stop, he just seemed to slide right past me and right through the car to my right (where I had almost been if I'd made my lane change). The car (Hyundai Accent) got stuck underneath the bumper of the truck, and the both started drifting to the left, cutting the woman ahead of me off. She had to hit the brakes hard to avoid being hit by the back of the trailer as the truck veered into her lane. I'd seen the truck hit the car and was already braking harder, and I came to a stop fine. The guy behind in a Ford Windstar me also saw it happen, and HE stopped fine. The guy behind him in the GMC 3500 van, however....
The 3500 hit the windstar at about 50. Pushing them both (10,000 pounds of van, now stuck together) into me at about 25. Pushing me into the car ahead of me. Total write off on a car I'd only had for four months. I did everything right, recognized a potential problem with that truck, gave myself lots of following distance to slow down if traffic slowed down, didn't rear end the car ahead of me when I had to respond to traffic (or the accident that could have distracted me), didn't brake so hard that the guy behind me hit me, but there I was on the side of the highway, still screwed because someone else didn't pay attention. It was dry, it was a sunny day, and I was doing much less than 80.
Driving "skill" often means nothing on the road. I wish more people knew this. It can definately help in a lot of situations, but it isn't a foolproof way of keeping you out of trouble.