I've always wondered too! The biggest issue with selling books on Amazon is that they're so strict with the Buy Box. Basically with all books Amazon win the Buy Box ahead of any sellers (even FBA sellers!) this means that to draw customers to buying their books, third-party sellers, often just sell at crazily low prices like you said.
Honestly I just don't think books are worth the difficulty of selling for new third-party sellers. It feels like it's too saturated a market, unless you've been around for years.
Also to do with postage. Amazon set the postage price for all media products (such as books, DVDs, etc.). Books are shipped by all Amazon sellers at a fixed price (at £2.80) which means sellers can't even set higher shipping fees. Again another difficulty making money by selling books on Amazon.
That's why I'd avoid selling them, unless you're well established on the marketplace and have super cheap suppliers.