Because one he beleives God gave him the strength to be who he was a bodybuilder. And with that came a relaxation of knowing what he had to do. And he was the most dominant Mr. O of alltime, and as was stated he was known for lifts as well as his work ethic. The reason why I said relaxation above, is because those who truly believe in God, Allah, or whatever, tend to have a quite yet jovial spirit about them, but when they know that there is work to be done, they are in the trenches.
The most jovial people I have ever met, have no belief in GOD, GOD believers to me are always child like, have trouble accepting reality and unhappy with what they have (thus the reason to pray), Psychologically speaking, the need to create a fictional father shows an immature attitude, a child like wishful thinking, the need to depend on something greater than oneself. Enlightened beings move beyond this stage. Surely you dont truly believe that a happy man would pump himself full of dangerous chemicals, to make himself as big and intimidating as possible, and then expose himself nearly naked to a bunch of strangers, all to entertain people on the fringes of society. Their are a lot of people who pretend to be happy, and then their are a few who are truly happy. Do you think those who flew planes into the twin towers were happy. These people had an immense belief in their GOD. Personally, I don't care for GOD, and what ever force created me, I am happy with what they gave me. I dont feel a need to pray to it to either constantly thank it or ask for help. I am sure that force is man enough to get along without my praise, thanks or desire for assistance.