Polygamy is actually very ineffective as a reproduction strategy for humans.

Polygamy is actually very ineffective as a reproduction strategy for humans.Not for women, not for "alpha men", and especially not for ordinary men.

Here we don’t even have to touch on the genetic diversity that is higher in monogamy and that our ancestors always strived for

Therefore, I will start with the fact that polygamy is not beneficial primarily for women, since in this case there is a need for a more extensive distribution of resources that have been limited for almost the entire history of mankind.

Human offspring are unusually costly to raise, and in polygamous settings, one man’s resources must be divided among multiple wives and many children, reducing per-child investment. Therefore, for women who want to become mothers, there is no point in relying on one "alpha male", but it is much more profitable to find an ordinary man who, yes, will bring fewer resources and attention - but all this will go directly to her and her children

And also for the same reason it is ineffective for "top men" because their resources will be divided more strongly and so even those children who are more "important" will not receive either the proper amount of resources or attention

And only after this can we talk about the disadvantages of polygamy for ordinary men who are not going to sit idly by in a situation where they will not get a relationship.
Nothing would have stopped them from simply killing all the "alphas" and/or taking their place since ancient times. And the only reason why "soft harems" and other similar structures exist now is the existence of laws, police and army that protect law and order

Young single men have also been the most reactive group of people throughout history and some of the most high-profile revolutions flourished precisely from their hands, such as the revolution in the Russian Empire and the Islamic Revolution