"That being the case, why should we automatically conclude that God did not also create life forms on other planets?"
Because it conflicts with some people's view of the bible, and therefore life, and renders everything they believe in questionable?
Instead of taking the wide view of creation as indication of the existence of God, some are only comfortable if it matches up literally with what the bible says - the earth was there and *plunk*, so was man, same then as he is today.
Agreed.
But I have yet to see where it conflicts with the Bibles view of people.