In that galactic society, a races' power and prestige are measured by what client races that have "uplifted", taking species that naturally evolved on those fellow planets and then engineering then with intelligence and directing their society. (To some extent in the other direction too: A race might have powerful patrons or grandpatrons that are still around.)

Anyway, point is that for the kinds of races likely to get their manipulators on a fresh planet, direct colonization is often way down their list of priorities. (And perhaps most of their population is already in a Dyson swarm or something.)

Also, the deep history of visits means there's been time for a lot of similar biology to spread around.