The Zhuanzhihui Patent Service Platform will give you the answer:
Because an invention can only be a technical solution.
Article 31 of the Patent Law An application for a patent for an invention or utility model shall be limited to one invention or utility model. Two or more inventions or utility models belonging to one general inventive concept can be filed as one application.
A design patent application shall be limited to one design. Two or more similar designs for the same product, or two or more designs for products of the same category that are sold or used in sets, can be submitted as one application.