Arrivederci, Flamethrowers. Can anyone explain why we have to finish every book we start reading?

There are some things I will never know: the feeling of zero-gravity, the constellations as seen from Antarctica, what happens to the narrator of Rachel Kushner's The Flamethrowers. As readers of these posts know, I am no averse to giving up on reading a book - I see no earthly reason to finish a book just because you started it. If someone does see a reason, please explain. But I would say I rarely get 80 percent through a book and then abandon it. Yet there was nothing further drawing me to this novel; I read through many wonderful, trenchant, idiosyncratic passages that captured the feel and mood and ambiance of a few different scenes - avant-garde art in downtown NYC in the 1970, Italian leftist radicals in a street fight against capitalism and authority, motorcycle racers on the Bonneville flats - and I kept waiting for a plot to take shape - and it never really happened. Things looked to get interesting during the Italian section of the novel, when the narrator was faced with some real conflicts and choices and when she truly appeared to be at some risk. But then we move back to NYC and more jabbering at art openings about a whole slew of minor characters whom I cannot keep straight nor do I need to. So I abandon this books with some reluctance because I do see so much potential in Kushner and hope she will write more and mature as a novelist, that is, as a story teller, and not rely solely on her great strength as a stylist. Proust can do this - I high measure to be sure - but none other. I realize that the plot may click together at the end, and I'm slightly curious as to whether the unnamed narrator is now looking back on her youth - from where? a position of authority in the art world? from a prison cell? - but not curious enough to plow onward. Arrivederci.