Usually we would say "if you think about it..." because the speaker is implying a statement that they've already given some thought to, and invite the other person to also think about it.
In some situations "if we think about it..." implies that the speaker also could use a little extra thought about something. Most of the time, people don't communicate this way. Someone is giving information, and someone is receiving it.
If I want to talk about dogs, I may say, "If you think about it, there are too many dogs in shelters. People shouldn't buy a puppy from a breeder." I'm indicating that I've already thought about it, and that's my conclusion. Now I'm inviting the other person "to think about it".
If we're in it together, the language might change to stick together in our thoughts. If I say, "If we think about it, how many billions of planets are out there? Why isn't it possible there's life beyond our own?" It implies that we can both sit and think about it, as it's something neither of us can fully comprehend.
In all, I think it depends on how the speaker wants the other person to feel about "the thought". The I'm-with-you-let's-think-about-this-together is a little less patronizing as it includes the speaker with their audience on the subject.