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But the real answer to your question is: why not?
Because it was tested in a lot of devices, proven to be the best width for readability during testing, and to justify it you're offering a sample of 1 wholly based on your subjective view with an argument that is roughly "Why not!" which is about as weak an argument that could be made for a change that would affect 300 web sites and 250,000 monthly visitors.
Why??
Because it's better, and I like more. For mobiles and tablets you give 100% of the space which is fine, for desktops you give an extra small surrounded by two rivers of dark grey.
Why don't let use the space available? But the real answer to your question is: why not?