Why do some managers lose their humanity towards their employees?

Some of them don't appear to have had it in the first place.

Some fear being outshined imho. I also think it's all about power. Some fear that if they are kind, their employees will demand more. Of course this is simply not true of good employees. I personally will bust my tail for employers that treat me right.

Some can't handle the pressure of the position. Especially when their own job is dependent upon getting the best performance out of the crew. They get chewed on by their bosses, and so they chew on those under them. The old expression that sh*t rolls down hill.

They hate their jobs too. They’re prob also not educated.

Power corrupts.

"Power tends to corrupt and absolute power corrupts absolutely." -John Dalberg-Acton

cause power is a drug.

Lazy scurvy dogs... The beatings will continue until morale improves!