They are, but there's a lot of lead time that goes in to shooting the pilot, and then even more in planning the rest of the season if it gets picked back up. They end up released at the same time, but that doesn't mean that a year hasn't passed since they started pre production on the pilot.

It's simply infeasible to adjust a show to fan feedback on a weekly basis I think. You'd need either a really small show, or one where most of the elements (sets, lighting, wardrobe, etc.) stay pretty consistent between episodes, then the work would just be a matter of tweaking the story, rather than figuring out how to get the whole crew out to all the locations required to make a traditional tv show.