I moved to AG simply because I like the brewing process and there's more 'process' with AG. I also thought I'd learn more about brewing if I made my beer totally from scratch...instead of using 'ready-made-parts'.
IMO...AG is a natural progression from extract. There's more to it and more to learn (in terms of 'hands-on' experience). That doesn't make extract bad or anything and certainly doesn't mean extract-brewed beers are bad. With many hobbies there are some that like to 'build' from kits of pre-made parts and just assemble them...but there are others that like to hand-make all the parts and then assemble those. Others are somewhere in-between. One extreme is a more advanced level of 'building' but so what...this is for personal enjoyment right?