There ought to be millions of civilizations in the galaxy
sending out radio signals. We don't pick any up. Therefore
civilizations are probably short lived rather than long lived.
We don't really know where the bottleneck is in the Drake
equation. (If you haven't heard of the Drake equation, it's an
equation used to estimate the number of civilizations in the
galaxy that are capable of communicating with us. The equation
is basically correct, but we have no idea what the values are
for most of the parameters. One of these parameters is the
average amount of time that a civilization remains capable of
communicating.) Using "reasonable" estimates people have come up
with a lot of different values. It's possible that the apparent
small number of civilizations out there is due to civilizations
being relatively short-lived. It seems to me that another strong
possibility is that there's a bottleneck in going from "life" to
"intelligent life." If I recall correctly, life on earth seems
to have appeared not too long