Why do only females of the species raise offspring?

The majority of species in the world are actually insects, and predominantly don’t raise their young at all. Those that do are normally hive based creatures, who often use male drones to provide most of the care of the eggs. 
Avian species mostly have both parents performing support roles.
The case of the question mostly applies to mammals, and the reason is that only the female has mammary glands, meaning that for new borns the division of labour is guaranteed to be the female providing sustenance. 
The primary goal of most species is to reproduce as many offspring as possible and preferably with different females for genetic diversity. In order to maximize the efficiency, males continue to mate and the females are left behind to carry the offspring.
The goal for the males is to make as many offspring as possible so fostering the children is simply not in their consideration.
Let’s consider humans. The top 150 fastest men are all still faster than the fastest woman. Strength plays out similarly.
That is just speed and strength. There are other advantages men have physically. All thanks to how we are built and our hormonal makeup. Women could go out and hunt and gather, but it would make more sense for the men to do it considering they’re built for that sort of action.
This is not to say men are superior. It is just biology. Women are more suited for raising offspring.

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