We can't ever know exactly why Google has chosen this layout, short of some Product owner answering this question. But some thoughts:
1) It's actually pretty common.
I checked Dogpile.com, Lycos.com, Yahoo.com, Bing.com, and Duckduckgo.com
They all do it:
2) Google is old.
I remember first being introduced to Google. Google was fast, and lean. But the results weren't always better than Yahoo's, and adjusting to the leaner search screen was a little weird.
Over time, I and mostly everyone else made the move from Yahoo, or whatever else engine. But no one enjoys changes like these.
Slowly we also began ditching our small 15 inch CRTs, or our larger 17, and 19 inch CRTs. Even those of with monstrous 21 inch CRTs moved to higher and higher resolutions on wider screens.
But Google always felt pretty safe and neutral. Even with the advent of iPhone and Android, as everything went to responsive and centered. The number of times Google has changed their search results layout so drastically I felt like it was a brand new search engine, is from my memory 0.
Even using the wayback machine, for 1999: Google looks like Google.
Of additional notes: At 1080 I can quite easily fit 2 browsers withe search results in a screen. So If I were writing a paper, and googling things I can do this with google easily and comfortably as is.
At the end of the day, my assumption it is as it is because
Don't change what isn't broken
1920 x 1080 and 1366 x 768 are much more common than very wide and very high resolution screens. For most users a sudden jarring change is more important I suspect to Google, than pleasing early adopters of huge screens. Especially, as I'd assume it's easier for Google to lose traffic from an older user on an older device than it is to lose an early adopter who buys gear expecting some odd behavior on some sites, as long as the site doesn't lose critical functionality.
Again, pure conjecture: But it's far too much for a comment.