Look, it is quite simple.
God is a busy chap and a hard worker. He has to be everywhere, all the time and he has to know everything - which means a lot of late nights reading and going online. One planet, one master-species (us), one set of geological sub-strata, one set of oceans, one global weather system.. it's enough for any deity.
OK, he made this place in six days and took a break. But he hasn't had a day off since. The guy seriously cares and he lives for his work. You can't say that of some deities. I think that Buddha fellow is rather too keen on his food these days and Allah (with all due respect to his prophet upon whom be peace and similar) frankly seems to have a slightly short temper, the way God did before he had a kid - but I'm getting off the point.
Also, none of the other gods made the world. It's in a book and is therefore true. Only God (as in God) can make worlds and he has no time to make and maintain another. It would be like expecting RvP to play for Manchester United and Stoke City.
How on Earth (to coin a phrase) would he (God) have the time to cobble together another planet, another range of flora and fauna?
Where would he get the time? The question is absurd.
Aliens? Schmaliens!