Yes, there are books that cross genres. The chances are you haven't written one. Or rather, books that do cross genres have an initial target readership and then get marketed to other readers. JK Rowling is found on in children's sections, although there is a big adult readership. There are a couple of authors who are probably genuinely cross genre - Kate Atkinson's recent books are literary but also detective novels, and I'm not sure where they get shelved, but she started out as a literary novelist. If she'd started with the detective novels, she'd have been shelved under crime like Louise Welsh, another very literary crime writer.
But I digress. The point is that you need to know very clearly where your main readership will be. If you don't, how can you sell to agents, and then publishers, and the marketing and sales departments, then the bookshops? You may not like the idea of being pigeonholed but if you get published that's what will happen. Publishing is a business, it's not an airy little hobby that a few people in ivory towers are toying with.
I know of one writer whose novel was picked up by an agent and, because the main character in the novel was a teenage girl, the agent announced that they were going to try to place it as a Young Adult. The writer said to me, "Whatever - if it gets published they can call it what they like." But it hadn't been written as a YA novel and it showed - publishers were interested, made suggestions but no one liked it enough to offer. The writer did several rewrites for different publishers and ended up so confused she stopped writing. (The agent disappeared too.)
I think she should have either stuck to her guns and said it was an adult novel or - if she'd decided it was right to go for the YA market - rewritten it before submitting to publishers. As it was, it went out neither one thing nor another, and I think that was why it failed.
I'm not saying you should write for the market - you should always write from the heart, and write what you want to write - but once it's done, you need to know where your novel might sit within the market. If you don't know, the easiest way of sorting out your genre is to think of writers you write similarly to, and see where they're shelved. Then read their blurbs and steal phrases for your synopsis and covering letter and focus your efforts on the best market sector for you.