Where is the closest livable, rocky planet to earth? One with a similar gravity, climate, atmosphere, water and natural resources, non-hazardous space environment, stable axis, and which is not tidally locked? We currently do not know of such a planet, but if we find one, it will be tens if not hundreds or thousands of light years away.
Suppose we were able to send probes to candidate planets. Once a probe arrived, it might take hundreds of years to send data back. If humans were then sent to colonize a planet, the journey would take thousands or tens of thousands of years, even at 10% of the speed of light. Meanwhile, no data about the planet would be available. Would our explorers find what they were expecting? How drastically could the planet have changed in the intervening millennia?
Our new arrivals may be in for a shock!