At least in the field of computer science, I have noticed this to be a common practice.
Why exactly is this done? I find it harder to read and follow.
There are some established standards for page layout in publications. One that I'm familiar with is a set of patterns for desktop (that is, personal) publishing by Andreas Rüping, published in the EuroPloP 1999 proceedings (University of Konstanz).
One of the patterns says that a page should be no more than about half text. A fair amount of white space, margins, and other elements besides text.
Another pattern says that a line of text should be no more than about twice the length of the alphabet used, so around 52 lower case letters in English. This isn't applicable to Chinese, of course. This limit is "imposed" by the physiology of how the eye works. Among other things, if a line is too long, then it is harder to pick up the start of the next line when scanning back.
But a full line of text in a reasonable font can be nearly twice that limit.
So, just those two patterns suggest two column format or some other accommodation.
And note that, if you are young, you have an easier time reading long lines than someone older with poorer eyesight.
I suspect it's not a matter of being "common practice" -- rather, it's the standard for IEEE and all of its associated conferences and journals (e.g., CVPR). Further, it's a sort of a nice looking formatting with plenty of infrastructure (e.g., .sty files), so it's a common choice when no particular format is prescribed.
There are plenty of other journals in CS that use a single-column format (e.g., SPIE, NeurIPS). But, this does leave a question of how many characters to allow per line. So, these single-column journals either (a) allow a large number of characters per line, which introduces some difficulty in reading, or (b) uses very large margins -- over 2" (on "letter" size paper). With respect to the latter -- perhaps this makes sense in Europe where A5 paper can be used (not sure), but in the US, I think this looks just awful.