I wouldn't say that people higher up can't be your friends, but when their office and them personally aren't the same thing.

I've seen family members fire other close family members, because a business is a business. Nothing special about it.

You can be friends with your colleagues, but when you understand that their role at a company is separate from your personal relationship - you may just get friends that transcend your employment.

My personal attitude towards employment - "I work for you while our goals align". Businesses most definitely treat any employee the same way.