The last great expansion was driven by the Empire. Since then, humanity has more than enough resources to sustain itself where it is; so there has been no need to expand.
Think of it as the same reason why there aren't houses all over the Australian outback.
There are not houses all over the Australian desert as it is rather dry. But I am sure rich corporations would be eager to take control of other earth likes all over the galaxy and put people there given that you have Anaconda sized ships that can get there easily and can carry enough in one trip to create a small habitat.