Except, if other people already have the same belief, won't it will already be priced into any oil futures prices.

So that only works if it's a belief you hold, but you also think that others believe the opposite.

With futures and options you have to be right about both direction and timing. If you trade on the belief that oil prices are going up significantly in the next 6 month, but it actually takes 8 month for the oil prices to rise significantly then you've still lost money.