"Almost every interface uses a fixed width and content is centered in the screen" is not a correct statement. This probably correct only for text content sites. Here are some examples of sites that do use full width:

• Pinterest uses the full width of the screen (at least the 1920 I'm using)
• Kizi - which my kids use all the time - also uses all the screen
• New Relic uses most of the page's width to display app stats.

And I'm pretty sure there are many more examples.

As states in many of the answers above, it is hard to create good text-based sites that use full screen width because of our human limitations. But when the site is not text-based, I see no problem.

You said:

Almost every interface uses a fixed width and content is centered in the screen

This statement is not correct. This is probably correct only for text content sites. Here are some examples of sites that do use full width:

• Pinterest uses the full width of the screen (at least the 1920 I'm using)
• Kizi - which my kids use all the time - also uses all the screen
• New Relic uses most of the page's width to display app stats.

And I'm pretty sure there are many more examples.

As stated in many of the answers above, it is hard to create good text-based sites that use full screen width because of our human limitations, but when the site is not text-based, I see no problem.