How many of those books exist? I've read a couple Sherlock Holmes books and maybe I'm too stupid but they didn't seem solvable. I know Agatha Christie novels are generally considered solvable. A quick search suggests Father Brown novels are solvable. I think there may be too much unsolvable books compared to solvable books so people don't usually try.

There's also plenty of reader engagement with complex literature since the reader can invest a great deal of thought in the themes and symbolism. Well-written prose conjures up incredible imagery that will find meaning in many, many things. Of course that's not the mechanical problem-solving OP talked about.