because rosalie had the surname hale before she became a vampire and wanted to keep it. at high school the story is that jasper and rosalie are twins so jasper takes the name hale as well
It's because to humans the story is that Dr.Carlisle Cullen and Esme adopted Edward, Alice and Emmet who are siblings, then adopted Rosalie and Jasper, who ব্যক্ত they were twins, so it doesn't seem like insest to the people who know they're all together, as in couples. Because if they were all siblings to the human eye it would attract attention and they'd be possibly discovered.