Actually it requires an older version of Qt5.
Calibre uses internal/private Qt headers, and needs the exact same Qt5 version it is built against.
So somebody would need to build it against Qt 5.8.0 from KDE:Qt5…
Calibre is not a KDE application, and not maintained by openSUSE’s KDE team.
But, because there are also some packages with similar packages in KDE:Extra, we decided to provide KDE:Extra built against KF5 and KDE:Qt5 too. And as we are aware of this problem with calibre, the plan was to add it to KDE:Extra too (as a link).
No idea why this hasn’t been done yet, it probably was “forgotten” because of other work (especially KDE Applications 17.04 was quite a lot of work recently).
AFAIK you should be able to install the rpm package provided by upstream though, this doesn’t use the system libraries AIUI.
https://calibre-ebook.com/download
Or switch back to Qt 5.6 as shipped in Leap 42.2… (which would mean you cannot have the latest Plasma5 and KDE applications though)