Isn't english known for not having an official set of gramatical rules? There are conventions and rules from universities i.e Cambridge vs Oxford comma. But there is no single body that is commonly accepted by all english-speaking countries to be codifying the official rules of the language.

It's a stylistic choice. Here's an analysis, complete with a link back to an old HN discussion:

https://www.pbjstories.com/blog/spaces-around-em-dash

I beta read for a popular american author and they started using spaces too.

That’s not entirely true. Some formats yes. In this case they’re using it correctly https://www.merriam-webster.com/grammar/em-dash-en-dash-how-...

I have seen this more and more and I don’t understand it.