Except for a token few, the cocks (the boys) are murdered because they are NOT worth caring for by those who exploit them for "us".
The hens (the girls) when spent--when they no longer "produce" eggs for "us"--are also murdered because THEY also are NOT worth caring for by those who exploit them.
Chickens, like all sentient nonhumans, are treated as comodities for "our" use. What else is there to know?...enough said.
*Peace and Health*