If we evolved to have multiples all the time we would more likely have extended families living together in 1 house. This would be better than the spouses by themselves for multiple reasons including money.
And during prehistoric times it would have been better for several reasons including that 1 would be more likely to survive than if it was born without any siblings.
Most mammals have litters of at least 2. An extreme of this is the cheetah which has litters of up to 9 cubs. They evolved to have litters because they were in conditions where 1 is likely to starve. Lions evolved this for a different reason since lions are not likely to starve. They evolved it because of the fact that they live in prides.
So for primates including us wouldn't it be better if they had litters often instead of single babies?