In the early part of the twentieth century, the pulps did in fact buy all rights to the stories. But years of writers fighting against that have resulted in better deals for the creators. Today, most fiction magazines only purchase a specific set of rights, including limited reprint rights.

For a writer, this is good. But as a reader.... There have been a few times when a magazine wanted to reprint a full set of their issues in a microfiche or electronic format, and were stopped by these contracts.