Because according to the detailed rules for notarization of wills:
Article 18 A notarized will shall be printed. The testator shall sign the printed notarized will after checking it according to the manuscript of the survivors.
If the testator is unable to sign or has difficulty in signing, the signature on the application, record or will may be replaced by seal; If the testator can neither sign nor seal, the signature or seal shall be replaced by a handprint.