|laziness, impatience, and hubris|
Granted, "showing up and voting out" does gain you some experience, but I seriously doubt someone would take the time to get up to fourth or fifth level without posting even once. Let's take a look at how long it would take, with the recent changes to the Voting/Experience System:
Just to get to level 2, you need 20 XP. With no votes, you can only show up. If you do this daily, you have a 25%chance at 2 XP. Given that, it would take 40 days to do that.
Next, you need 50 XP (30 more) for level 3. With 5 votes a day, and a 25% chance to get a +1 with each vote, I calculate it'll take another 18 days.
But, we're still not there. We need 100 XP (50 more) for level 4, with 8 votes a day now. That's gonna take another 12 days.
Total that up, and we've got 70 days. And that assumes you log in every day. Shoot, we're still not to 'monk' level (for those of you keeping score at home, it would take another 23 days to do that). I doubt someone is going to make that kind of commitment, just to see a few job postings.
I think it's far more likely that the user will post, vote, and, in general, contibute to the site. In fact, simply the act of voting (wisely) is contributing to the site.