Primarily because as yet I haven't read any of them and they mock me continually with their frequent appearances on lists. Even worse I am unlikely to ever read most of them so they'll continue to disturb me without mercy.
A couple of further points. Whenever I see certain icons, such as one for Richard II (unread by me), I tend to see it as Richard III, which I have read, and gleefully anticipate checking the item off, only to be disappointed on closer inspection. And no matter how many books you or I read we all know that others will arise from the mire to niggle us.
With cruel intent I've deliberately chosen one of the most popular and overused icons for each title, so as to increase nausea symptoms. If that's too much for you, try my earlier gentler challenge at the link.