As a web developer speaking for the sites I've made: we didn't design for large width display. And white space is what defaults when you scale the browser width up (or whatever the CSS background color is).
The costs of designing and developing a responsive and scalable website across desktop and mobile are already quite high. Our analytics show that 90%+ of our users view our site in <1400 px wide resolution.
At the end of the day, it's not worth it to spend the time and money designing and implementing for the <10% of users who browse our sites at >1400px wide resolution.