So many reasons!
Firstly, I just love books. I don't mean just reading and collecting them, as actual objects I find them so beautiful. I love the variety of sizes and colors, the way they look on a shelf and the way they feel in my hands, and when you're holding a well made book you just know. I love seeing how printing techniques change over time and how those techniques can add to or detract from the quality of the book. I love paper, I love colors, I love art, and I love reading, and books take all of those individual components and somehow make them bigger and brighter and special.
I also love antiques, crafts, and folk art. Wood carvings, metal engravings, quilts, hand made furniture, on and on the list can go. I'm not very crafty though and some things take a lot of skill, know how, and often expensive tools to start. With books, the entry level cost is quite low and the initial learning curve starts out relatively low (but ramps up quickly the better you want to get and the more you want to learn and incorporate). While I wouldn't call my books "art," I would say I take a deliberate and artistic mindset when I'm making them. Coupled with the fact that there is learning and research and problem solving involved in figuring how to make a good, sturdy book that also looks nice, there's growth and satisfaction there.
Lastly, I tried to get a job in bookbinding, but unfortunately everyone that is hiring for that kind of thing requires degrees in stuff such as library science or art conservation or, for everyday publishers and manufacturers, there really isn't demand for traditional, handmade books. So I figured one day, if I really wanna make books, I should just make them on my own, because if I love them so much, why the heck not?