Because the document at that link can change. What can work is if there is a hash of the data, and you hope that the data is still there at the link. Better if you have the actual data that was signed. Which means, we're back to actual documents.
What we SHOULD have, however, is a github for lawyers.
The document at the link is a PDF, so the "the document is changeable" problem could be addressed by downloading the PDF and attaching it to the email agreement to serve as the record copy.
TFA also points out that the document is at the Internet Archive: https://web.archive.org/web/*/https://waypointnda.com/3.0.0
EDIT: Each party presumably will have email records with the PDF attachment — and if the attachments don't agree, then the parties' lawyers will be busy. But again, how likely is that to happen, and how much time and money do you want to spend to prevent it? At some point you hit diminishing returns (and OCD behavior).
It was a really great talk. Not sure if there is an English version somewhere:
Seems to be: https://media.ccc.de/v/31c3_-_6558_-_de_-_saal_g_-_201412282...
Direct: https://cdn.media.ccc.de/congress/2014/h264-hd/31c3-6558-de-...
Which has german/english dual audio, but defaults to german (so presumably google ruined the english track on upload). If your video player doesn't support picking audio, you can probably fiddle with ffmpeg to strip out the german track.
One of the advantages of a fixed reference document is that, apart from confirming that you have that document, you don't have to read it to know what it says — and to have a sufficient degree of comfort that the other side isn't trying to sneak something past you.
In contrast, if Party A (or its lawyer) copies and pastes the contents of a reference document into an email, then Party B will have to read the thing, which will take time and, if a lawyer is involved, cost money in legal fees.
An analog that comes to mind is the INCOTERMS rules for shipping goods: If you agree that the goods will be shipped DDP (Delivered Duty Paid), or EXW (Ex Works), or whatever, then you know what you're getting, without having to read the rule again to be sure the other party isn't trying to slip something in on you.
Another analogy is arbitration: If you agree to the American Arbitration Association's rules for commercial arbitration, then all concerned can go to the AAA Website to get a reference copy.
Remember who we're talking about here: Lawyers and business people — too many of whom don't even know how to use Microsoft Word "properly." [0]
(You have no idea how many contract drafts I review in Word where the drafter used 'Normal' style for everything and just bold-faced the headings; used blank lines to put spacing between paragraphs; and didn't know how to do automatic paragraph numbering.)
[0] Obligatory link to the famous comment about Dropbox's launch: https://news.ycombinator.com/item?id=9224
IME it seems to be trivially easy to have things removed from the Internet Archive.
>How can I exclude or remove my site's pages from the Wayback Machine?
>You can send an email request for us to review to info@archive.org with the URL (web address) in the text of your message.
https://help.archive.org/hc/en-us/articles/360004651732-Usin...
It used to be trivial to hide existing content in their index from being viewable (for your domains). They entirely changed that a number of years ago.