In my early 20s, I had an awful experience hitch hiking. I'll never do it again because I seriously thought I would not live to see the next day.
During that time, I knew quite a few people who hitch-hiked long distances. Every single one of them had experiences where they thought they were going to die or sensed they were is serious danger. Don't get me wrong, 90% of the experience is positive and interesting, but that 1 / 10 ride you get will scare you straight.
The numbers may suggest that hitch hiking is safe, but the numbers don't, and cannot, tell the stories of those that had good reason to be traumatized by a bad experience. As an experiment, try hitch hiking across one entire state. I would gladly offer my $10 to your $7 that you wouldn't want to do it again.