The bag is designed to keep that food fresh for a year or more, and when you are done with it, you are throwing it away unlike a container which will need to be washed between bags. Keeping it in the bag also ensures no spills during transfer, and as the bag goes empty you can "burp" it for a tight fit with an appropriately strong chip-clip.
If the bag itself isn't too large or heavy for you or where you need to keep it, then if you need a container for it just for protection or what not, I'd recommend scooping from the bag, and just putting that whole bag in the container. That way you don't have to worry about washing the container at least, since its not coming in contact with food.

The other option, is if you get an automatic feeder like the
Petmate Le Bistro
, it can fit 10lbs of food which is the average larger size of most brand's bags. So just fill it up, and when it runs out, throw it in the dishwasher before refilling. From what I can tell, it seems to have a pretty decent seal, although not airtight.