Why are biglaw interviews so long?

Genuinely curious, in a profession where time is the most valuable asset, why do firms spend so much manpower to interview candidates? I've heard of callbacks being 6 hours in total at some firms. What do firms get out of these interviews that they wouldn't get in an interview of normal length?

Also, why is there so much focus on interviewing in general, when the job is really just sitting behind a computer screen and reviewing documents, doing research, writing, etc.? I've also seen firms ask about what you like to do for fun--why do firms care, when realistically you won't have that much time for your hobbies?