The first question is where are you looking? If you are interested in a particular studio, outside of industry sites the best place to find jobs for the studios is on their own site.
In my experience (from just looking around, speaking to people) the industry has no shortage of applicants. Because of that, there is no real need to expend extra resources paying a premium to get listed on various jobs sites. People interested in their companies will already be checking their website.
Finally, the experience issue. The positions that you see advertised on these sites are generally not for entry-level positions.
Larger game companies (like the ones you listed) get tons of entry level resumes sent to them from a variety of people. There is little financial incentive to advertise that they are looking for entry level people.
Also, I've yet to meet a recruiter that wasn't ok with having my resume on file. It's generally understood that comapniescompanies need fresh blood.
What isn't always known, is if they have any place to fit the higher experience people (at their right pay grades and all) so it's beneficial to advertise it. Also, these people are generally harder to find, and might be otherwise employed.
These people are less likely to shoot resume's into the dark.
Of course, making a cool demo never hurts.
For more information on applying for a job in the games industry, I turn to Sloperama: http://www.sloperama.com/advice/jobapp.htm