30" 2560x1600 monitors are super expensive still, 2560x1440 monitors are marginally cheaper. There are only two single-monitor resolutions above 1920x1200/1080, and that's one of them, and they are both close.
Sites test it to show 27/30" monitor users what performance is like at that res. Just because not everyone has a supporting monitor doesn't mean much, especially when you are talking about up to $500 graphics cards.
Most websites do multiple resolutions, including the more common ones like 1680x1050 and 1920x1200/1080.
2560x???? pushes cards to their limit, which is also what people want to see.
I don't know where your "most benchmarks are based on this" idea comes from when websites cover multiple resolutions, and that's just one of them.
Don't care about the results at that res? Then ignore them and focus on the relevant results at lower resolutions.