Calibre, at its core, is a database. As such, the best view into the database is a list. Why do people hate lists? The last thing calibre needs is a faux-wood bookshelf view (which doesn't even make any sense -- who puts their paper books on bookshelves with the cover facing out?).
Also, keep in mind that calibre is cross-platform, and developed by a very small set of people. Its UI is designed to work everywhere, which means it's not necessarily the prettiest anywhere but it gets the job done. If the developers had to maintain separate GUIs for Windows, OS X, and *nix, 90% of their time would be spent on keeping each of the different GUIs at feature parity and fixing bugs in the rather than focusing on the important stuff like device support, conversion functionality, etc.