Why didn't humans evolve to reproduce identical twins all the time?
What you describe could have happened under the right conditions. However, there are a few things you haven't considered.


Because humans are especially altricial, always having twins would double the cost of children on parents.
The benefit of sexual reproduction is immune diversity. So a population like this could be far more vulnerable to disease.


So as far as this thought experiment goes, genes can't be too selfish or they dramatically reduce their own fitness. It's something worth simulating computationally to see what happens.

References


Sexual reproduction as an adaptation to resist parasites (a review)