I tend to feel the same way but here's the problem.
Several books that did not seem very promising for like over half their length turned into some of the best I've ever read. In fact my personal favourite book, Imajica, I think if you had a 50-page rule an awful lot of people would drop just little bit before it really caught fire.
There are also book series where like, they have one weak book, then they're back on form. Rivers of London is a prime example, I would argue - and it's had more than one "miss" - in some cases they don't become obvious as a miss until fairly late on (I'm looking at you, Whispers Under Ground). But there are others, deceptively similar where they just sloooooooowwwwwwwlllllllly goooooooo downhiiiiiiiillllllll. Looking at you Stormlight Archive. I should have quit the moment Kaladin was called Kaladin and was kind of acting like a Paladin the moment that goddamn bastard fairy (god I hate that fairy) gaslighted Kaladin into not doing a slave revolt but in the same book they actually managed to flip something so I thought things might improve. Luckily in book 3 the horrible multiple-chapter diversion into "a story it was literally impossible to understand without having read another book trilogy from a different setting entirely" pushed me over the edge.
So I give books a little more rope to hang themselves. The positive result of this is that I am often able to really deeply insult some bad books because I know exactly what is wrong with them in some detail! Silver linings etc.