When you cryptographically sign something you need to be sure of what you are signing. That means that you have to be able to look at the document and see each and every character that you are agreeing to. So in a future where we actually work out how to do cryptographic signing we will be forced to use something very close to plain text. You need a format that will not unexpectedly change on you sometime in the future. That might break the signature or worse, maliciously change the meaning of the document without breaking the signature.

Opaque blobs like docx are not suitable for applications where the content of the document has to be completely clear to the various competing parties involved in something like a contract. It only works because the document gets printed out and then signed with a pen. If we want to move past that we need something different.