I think they use standard 18650 batteries (might be wrong) and there have been discussions from Tesla saying that the round batteries also have the benefit of a

I think they use standard 18650 batteries (might be wrong) and there have been discussions from Tesla saying that the round batteries also have the benefit of allowing an air gap to insulate and help mitigate overheating problems. Plus they are mass produced, cheap, and have a relatively high energy density. The new factory might work on creating some new battery technologies though, not sure. Ideally yes, you want the battery as compact as possible and lots of round batteries aren't the most efficient for that, but when you factor in cost and the natural heat insulation, it might have been the best option at the time. If it works, roll with it and work on one of the numerous other problems they have to solve.