Its really not, only Linux makes it that way. If its done right (as it is on mac, and to a _very_ slightly lesser extent, Windows) there isn't 'blurriness' at all. One monitor will just happen to be sharper than the other.

I work every day with a 4K monitor and a 1080p monitor side-by-side and it works well. Linux couldn't handle it in any configuration, especially with mixed Intel integrated+nvidia graphics, but Windows is a champ.