One more possibility - as human populations get wealthier and more educated, their population growth rate decreases. Maybe by the time lifeforms get the techno

One more possibility - as human populations get wealthier and more educated, their population growth rate decreases. Maybe by the time lifeforms get the technology to travel to other stars, they find that they don't need to expand. There are many places (even within the USA) that we have the technology to colonize, but not the desire.