For me, it's because I LOVE to take pictures, and the process of making images.
I learned photography in the 1970's with B&W and a totally manual Canon TX. I love all of it, which includes deciding on the film speed, deciding on what aperture will give me the depth of field or subject isolation I want, deciding on the shutter speed to capture movement or not, deciding exactly where I want the lens to focus. This also includes deciding on which developer, processing the film myself, printing the images (now more often scanning them). I love doing all these steps, and taking the time that each step requires.
Now I use a Nikon D4 for work, because my editor doesn't give a rat's behind about the process, she just wants the images, properly exposed, and in a matter of minutes.
But for my personal work, I love the process of making a photograph.
When I was young and first learning this artform, we used to talk about the difference between photographs (the things Ansel Adams made) and "pictorial representations of what was going on" which is what I saw from so many "auto everything" cameras.
Best,
-Tim