Originally posted by elementXI:

Ultra-wide monitors...

It doesn't matter. Nobody makes the content width big on websites because it makes readability worse.
Don't like unused space? Exit fullscreen.

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The optimal line width for web design, in terms of readability, is generally considered to be 50-75 characters per line (including spaces). This range ensures comfortable reading without requiring excessive eye movement or straining. While longer lines might be faster to scan, they can hinder comprehension, and shorter lines can disrupt the reading flow.

50-75 characters (including spaces):
This is the widely accepted sweet spot for body text on websites. It allows for a good balance between ease of scanning and focused reading.

Why not longer?
Lines exceeding 75 characters can cause eye fatigue as readers struggle to track the text, find the next line, and maintain focus.

I have seen many users requesting that Steam accommodate the empty space of displays with large resolutions. As someone who uses a 3440x1440p display daily, I don't think this feature will benefit you in a meaningful way.

A toggle switch that would manipulate the width of the user interface would add unnecessary complexity for the users and developers. Regardless of what displays we use, expanding the width of the store page content would cause more eye fatigue.

If you don't want to waste space then you need to resize the Steam client window.