Gamblin Mat Cauthon wrote:
Rob recently gave an interview where he explained this design choice. To paraphrase, initially buildings stayed from game to game but it caused runaway problems. Buildings provided so much advantage that players in early games cared more about buildings than actually winning the current game. And once someone had a lot of buildings they became too powerful. So the buildings were nerfed, but then there was no incentive to buy them. So Rob decided that buildings don't carry over from game to game.
You nailed it. During one design iteration I decided to play almost purely economic. After a few games I had established such a significant degree of upgrade (purposely ignored winning to buy buildings and upgrades) that not only was I likely to win most games, but I had built an impregnable defense. Good times