It is not mentioned in Spider-Man's trilogy, but it is gradually revealed in Fantastic Spider-Man. Others failed to fuse with the spider's genes, only peter parker was perfectly fused with the spider's genes after being bitten. The reason is that peter parker's father, Richard Parker, is the chief genetic scientist of Osborne Company. When he learned that Osborne wanted to use genetic technology to make biochemical weapons, he injected his DNA into spiders. In other words, no one can get the spider's genes except his DNA and his blood. Richard Parker was framed by Osborne Company and killed with his wife on the way to escape.