1. I'll level with you. It's an insurance thing. (Same with robot butlers, but for different reasons)
2. Let's compare cows to pandas, shall we? Cows are big, dumb and tasty, Pandas are big, dumb and not particularly tasty (If you squint, they both kind of look the same too!). Now, I don't have the exact numbers on hand, but I think there's several billion cows worldwide, whereas there's only about 5 or 6 pandas. You take being eaten away from cows, then what have they got? Making Milk? Any mammal can do that, even Pandas! (if we milked people, I think a lot more people would want to be farmers!)
3. Actually, there probably is oil on Mars. A lot of scientists speculate Mars used to harbour earth-like life, so unless someone else has already stolen it, there'll be oil. Of course, the energy cost of moving oil from Mars to Earth is much greater than the energy that could be provided by said oil...
4. Dogs are relatively easy to domestic and breed, bears on the other hand...