I have always asked myself the same question. Here are two things I think:
Maybe it could be that God created such a beautiful universe(that is huge) because he knew that we would soon need to expand onto other planets because of over population, or what ever the cause.
Or, it could be that we are not alone, and the Universe is full of planets that have life on them, but we just don't have the technology to move on.
But, one thing I don't get is why people think aliens are going to be these big blobs of what ever they are and are mean and green. Why wouldn't you just think that they are regular humans like us. I mean, God create all humans in God's image. If you wanted to argue the point that aliens are not humans, but animals, then why would you call them "intellegent" lifeforms. If they were animals, they would be running on instinct, not knowledge.
So, the point that some go after are that "Jesus said that aliens don't exist"(I don't know where that was in the bible, it might be). To the point, maybe these aliens are not aliens. We are one big community of people that live in different parts of the universe. I mean, the meaning of alien may differ from what Jesus's meaning of alien was.
So, it is an ongoing scientific and religios debate. I would say, well yes there are other LIFEFORMS out there, I wouldn't call them aliens, persay.