Because it just so happens that this is where we are.
Earth can only sustain so many people, and since we do not currently have a viable way to get off this earth and/or to terraform other planets (hell, we can't even agree on a common plug to use for household appliances) it looks like it will remain that way for a while.
Um, we've left the earth already. The only reason why we don't continue to do so is that it isn't cost-effective -- which means that we have another resource-allocation problem rather than some mysterious insurmountable challenge.
Earth can only sustain so many people, and since we do not currently have a viable way to get off this earth and/or to terraform other planets (hell, we can't even agree on a common plug to use for household appliances) it looks like it will remain that way for a while.