Blurry on a printout, or blurry on your screen? There is no fix unless you can control what Word and Powerpoint choose to do with the content on the clipboard. Testing below was done with Preview on macOS 11.6.1.
When you use that rectangular selection tool and copy that content to the clipboard from Preview, it resides on the clipboard as a PDF and a TIFF image. When pasting into the Microsoft applications, and depending on how they are written to accept clipboard content, the object that arrives in those applications may be either PDF or TIFF. The TIFF image will likely be 72 dpi, though the PDF content would be resolution-independent unless the Microsoft applications are converting PDF into 72 dpi jpeg or png during the paste operation.
I don't have MS Office here to further test this.