What % Zoom do you have your browser at? When I have mine at 100%, it fills my page. Only when I have it at a smaller % (which I usually do because I like to be able to see more content at once) does the space on the sides get bigger.
I will add that it also depends on your screen width. My above comment was based on my laptop screen. On my wider desktop screen, there’s still some space on the sides at 100%.