b/c the system is set up so that if a PI doesn't look at your application, you essentially won't be considered and you'll get an automatic rejection. The best way to get a PI to consider you is to contact them and tell them about your interest in their lab. Like I said, many labs have already filled up, sometimes from returning students. Some labs only take college students, which makes it even harder for new high school students to get a position.
If you look in the FAQs or something on the application site, the instructions say something to the extent of "we strongly urge you to contact a PI if you are interested in their lab"....so they're actually pretty explicit about it. There's a list of the institutions as well as links to their websites, and you can probably find directories of PIs on those sites.
fyi- getting internships, no matter what the company/institution, is always like this. you have to make personal contacts, or else you're just another applicant. *steps down from soapbox*
hope that helps