We will settle the galaxy for the same reason we ventured out of Africa. It is who we are. For the same reason the Greeks build city states at every location their ships could reach.

80.000 years, if it even takes that long with technology to come, is not that much.

Automated machines can prepare new star systems for settlements. While humans might be born out of stored DNA upon arrival. Or whatever we will look like at that point in time.

An alien civilization that follows the laws of evolution would have been detected by us already, at least in our galaxy. 1 million years is little in cosmic time, but gigantic in exponential growth what we will accomplish.