Makes sense. Most people I know usually work during the school year / summer as research assistants under a professor in their lab before going on to intern somewhere. Unlike with computer science, with more hands on type of sciences like chemistry, I'm sure having some lab experience is pretty important if you want to be a meaningful contributor during your internship.

I don't know much about how job postings online work, but quite a few companies come recruit during the school year, and all of the events are open to everyone as long as you're a student. So I'm not sure it's necessarily as closed off as you're making it out to be with having to be contacted on an individual basis.