Because I don't want to spend all day resizing my browser when I switch tabs?
I want websites that are made to be read to optimize for legibility and comfort, not for "not wasting screen space."
As I type this comment on HN, the right hand side of the text box where I type my comment is entirely empty.
Should the text box be full-width so it fills the screen and doesn't "waste space"? Hell no. It would be a pain in the ass to read my comment as I type it.
I don't know about you, but I'm very seldom (never?) reading more than one column of text at a time. Therefore, as I'm reading, anything else on my screen is "wasted".
Doesn't matter if it's white space or anything else.
It seems like we can't avoid "wasting space."
To see the absurdity of that metric, why don't you just make your window as small as possible all the time so you're never wasting any space? Why don't you reduce the font size to 1 pt so you don't waste space with fonts that are comically large?
I must admit that logic kind of escapes me, since it misses the most basic purpose behind reading on websites.
Now, I'm not unfamiliar with the regular HN comments about how everything is "dumbed down" and it's all about "information density" and cramming everything in one screen should be the be all and end all of design.
Unfortunately that opinion also seems to emanate from people who can't substantiate their preference with anything else than appeals to "power user" aesthetics, with very few actual use cases justifying it.
These are mostly ego-based arguments I tend to dismiss when discussing design decisions.
What does a screenful of text allow you to do that you can't do with a better proportioned column of text?
Unless you're the Flash, in my experience, absolutely nothing.
As far as saying the experience of reading properly should be left to the user/reader, I simply disagree. I'd rather benefit from the skills of designers and typographers presenting the content for me in the best manner than relying on hacks like resizing my window to get a decent experience.
I don't expect to go to a restaurant and be thrown ingredients for me to customize them _my_ way, I want my food cooked deliciously. And I want my content formatted properly and elegantly.
P.S. Note that your argument is entirely symmetric: if you don't want to "waste space" on a fixed width website... why don't you resize your window?