If you've tried using different cameras, you might've noticed that the manufacturers use their own proprietary RAW format (Nikon’s is .nef, Canon’s is .cr2 or .crw, and Sony is .arw).
"Proprietary” refers to the designer or manufacturer's rights for controlling a product. Each camera maker can refuse to share the information that explains how a file is processed in-camera. If, in the future, these companies disappear, their proprietary files may prove difficult to read.