You must be referring to the constrained width of the page, you can change that in the preferences if you really prefer having the text cover your whole screen.

There's a reason we have a character limit per line when writing code - it's much easier to read and understand short chunks of vertical text. Personally, I find it far more comfortable reading text which doesn't go above 600-900px in width and this has been a well known UX/UI guideline for the last 10 years. Having your eyes go from the far left to the far right of your screen is also very straining on any >21" screen.

These kinds of objections are predicated on the idea that white space is inherently wasteful.

But the real question is, why is it good to strain the eyes darting them across distances that large. What is gained by filling in the white space just to fill it?

Yes, though it also contributes to that "busy" look if you try to cram as many things as possible on the screen. The content might have a logical separation but spatially multiple types of that content would still be close to your focal point which makes them a distraction.

We live in an age where attention is a valuable currency and most people would likely leave your site if they can't quickly find the thing they're looking for. The fewer things you have on your screen at a time, the easier it is for them to find what they're looking for. It doesn't apply everywhere but I think it applies to a good amount of cases which is why lots of white space has become the norm.