Because publishing goes hand-in-hand with copyright. 98.65% of the time, the publisher will keep the copyright to the works they publish. In fact, the only time I've seen it otherwise is when the composer is such a big name that he has a huge amount of clout behind him. An example I can think of is Aaron Copland owning his own copyright while Boosey & Hawkes publishes it. (I speak, of course, of original pieces, not arrangements which are different.) To put it plainly, always be wary of a publisher saying that *you* get to keep copyright; they're probably selling you something.