Where do you think the idea of "sameness" came from, and why were there no differences in the people other than gender and assigned jobs?

I believe that the "sameness" came around because of a desire to make sure that A) people would not be sad and B) that people would not make bad choices.

If people are not all the same, then some people are going to be better than others.  They will be more successful or have smarter kids or make more money.  This will cause dissatisfaction among many people who will be jealous of those who have done better.  With the sameness, this will not happen.

With the sameness, people will also not make mistakes.  They will not choose jobs that make them unhappy.  They will not have too many children or get divorced.  This will make the whole society run more smoothly.

There are no real differences between people because the community does not want these things to happen.