Thats because its default artboard size when you are creating new web project in XD and i supposed that this will be the best choice.
And also i don't designing it for myself, but for clients which i don't know what size of monitor they have 
It doesn't even matter what resolution screen your clients have, it's what users of the site will have. I think either 1366 or 1440 would be a better choice (and a better default for XD) because those are more common desktop screen resolutions (and you won't have this issue). Ultimately when the website is coded it will have to be responsive (meaning it adapts to different size screens), but for initial designs in XD, I'd go with:
- 1366 or 1440 for desktop
- iPad (a 768 tablet)
- iPhone 5/SE (because it's smaller and you need to make sure your design can work on these types of small devices)
It's harder to make things fit/work on smaller screens with limited space. If you make sure things work on smaller screens, you can then work up to bigger screens.
I know that's not specifically answering your original question, but I hope this advice helps you (and others).