While it is wasteful, I think the readability gain is far from nebulous. If you look at newspapers, they break pages up into columns. If books exceed a certain width, they get broken down into columns. Journals often break their articles into columns.
Now, a fair follow-up to this is that why don't we make text on the web into columns, so instead of big sidebars of white space you fit multiple columns of text on the screen? I'd say this is probably because the web uses scrolling instead of pagination. If there's more text than can fit on the page, it's faster and easier to just scroll down than it is to load a new page. But if you have a layout with columns, you'd have to scroll down to read, then scroll back to the top each time you finish a column, which is a hassle. So just having one long column requires the least user effort.
All said, of course some people might have different preferences. I'm sure some people find it easier looking from side to side with full screen text than scrolling down, or easier to click a next page button than scrolling down. But I think the trend for fixed width single columns on the web is one that's meeting many if not most peoples' preferences.
I took several graphic design classes in college, and the rule burned into my brain is "between 8 and 13 words per line" for maximum readability. This was a guideline for print, but I do think it matters on the web as well, and it's very easy to exceed!
Now, a fair follow-up to this is that why don't we make text on the web into columns, so instead of big sidebars of white space you fit multiple columns of text on the screen? I'd say this is probably because the web uses scrolling instead of pagination. If there's more text than can fit on the page, it's faster and easier to just scroll down than it is to load a new page. But if you have a layout with columns, you'd have to scroll down to read, then scroll back to the top each time you finish a column, which is a hassle. So just having one long column requires the least user effort.
All said, of course some people might have different preferences. I'm sure some people find it easier looking from side to side with full screen text than scrolling down, or easier to click a next page button than scrolling down. But I think the trend for fixed width single columns on the web is one that's meeting many if not most peoples' preferences.