I would say that there are at least two good reasons. A4 is not a relevant standard in the biggest market (USA) and A4 proportions are objectively not great for reading (too elongated).
I spent the first 30 years of my life in the UK and US letter paper always looked too squat when I encountered it at work. I've spent the past 11 years living in the US. I recently dug out some old papers on A4 and they looked weirdly elongated. It's surprising how strong the perceived weirdness of paper dimensions could be and and how easily it was changed. Seems like it's totally subjective and we could probably be comfortable reading in many proportions.
It's a workable compromise which ensures that all versions of a document have the same pagination/index.
I still think it's odd to not translate one or the other if describing a 2D size, though.
Type:
11*25.4
279.4To me A4 looks weirdly fat/short/square-ish on first look.
On a longer look I think it looks very nice (especially so knowing why the ratio was chosen).
In general though, I prefer narrower columns for reading (closer to A5), so I don't know why it should be even wider/shorter.
My understanding was that the advantages of A4 are more practical than aesthetic. Is that not true?
The oft-cited reason is that you can fold A4 in half, etc. to form other standardized paper sizes.
Which sounds nice, but in practice just doesn't come up all that often.
Papermakers definitely benefit from this standard, which indirectly benefits you.
And from personal experience, being able to turn any stack of A4 papers into an A5 'zine is great.
Photocopiers and printers outside America include buttons for this.
It's a standard feature of photocopiers in the US too.
This statement is purely subjective. It is false in general and there's nothing objective about it. I've read thousands of pages of A4 with no problems, and in fact I'd say it's great.
This subthread is about aspect ratio. dfadsadsf claimed "A4 proportions are objectively not great for reading (too elongated)".