That's one reason, but seriously there is another that has just come to light:
I can't remember who, but someone came up with a formula a few decades ago for the number of intelligent species in the universe. It was based on the number of galaxies, the % of stars like ours, of the same size, number that had planets, planets that were the right size and distance from the sun. Even using the lowest estimates, you still come up with a big number. They then cut that down to intelligent life and those able to make radio broadcasts.
It was this idea that began the whole SETI program.
The only problem is that on Earth radio broadcasts are actually falling now. Since everyone is going for cable and direct broadcasting we are not sending out 'I love Lucy' into space anymore. This reduces the window when we have been broadcasting into space to about 100 years. That reduces the figure in the formula by a large factor too.
There may be life out there, but we may never detect their presence.