Well we shouldn't assume. The two dominant hypotheses for the developmental trajectory of intelligence include "expansion" and "transcension". I'm actually reading about transcension right now. I find it quite plausible and worth further investigation.

From the John Smart article I'm reading:

The expansion hypothesis predicts that some fraction of advanced civilizations in our galaxy and universe must become beacon builders and spacefarers, spreading their knowledge and culture far and wide. Expansion is the standard expectation of those engaged in SETI and METI today. Expansion scenarios typically assume ETI messaging to be bounded by the speed of light, and space travel to occur at some significant fraction of the speed of light.

By contrast, the transcension hypothesis, also known as the developmental singularity hypothesis proposes that a universal process of evolutionary development guides all sufficiently advanced civilizations increasingly into inner space, the domain of very small scales of space, time, energy, and matter (STEM), and eventually, to a black-hole-like destination, censored from our observation.