AFAIK This is mostly because in traditional grammar/typesetting there was never any use case for double dashes, but there also was no way on keyboards to type an em dash, so in the aughts all the text editors and OS developers decided to make their software auto-replace two dashes to an em dash as the easiest way to get the em dash.
There was a brief period in 2008-2012 I feel like where smart phones and typing lots of text for non-casual interactions blew up in popularity, and people started just seeing and using double dashes because they were used to it, and then I guess people started expecting the double dashes to stick around, and got jarred when smart phone apps caught up to desktop capabilities.
I guess this never hit the radar of the people who invented Git (or even CSS today with its variables), who thought "yeah dash dash is a good idea, that'll never cause a problem".
The whole double dash thing goes back a lot farther than that. The 1970s in fact. It’s all from Unix.
Here’s a great history
https://blog.liw.fi/posts/2022/05/07/unix-cli/
By the time Git rolled around, command line options were pretty standard (for decades), along with libraries for parsing them.
Even once operating systems started doing “smart” shortcuts, these standards never changed. Note that Terminal/Console/Command Shells NEVER do any modifications of typed input.
The real problem came when people started blindly copy-pasting or typing g command lines into word processors, blogs in browsers, Slack and its ilk, and lots of other places that did do “smart” cleanups.
Blaming Git is reversing where the blame lies. The problem is in the design of smart typing features that never considered this case.
Though note that sometimes they do take account of the differing requirements, particularly if you use monospaced or “code” layouts.