Because we haven't gotten a way to get fuel outside earth yet (that I know of), or a solution to long travel times. As such we are practically still bound to earth, which is finite.
If we're willing to deal with the (technologically but not politically manageable) consequences, nuclear rockets are a proven technology and can reach most of the solar system within a year's traveling time.
This is a bit of a non-issue. Expecting to be able to travel anywhere within a matter of a few dozen hours is a recent phenomenon.