Pet peeve

Why do people answer their second line when they're on an important call, and then when they answer, immediately ask if they can call you back? They can't stand to let a call go to voicemail?

I know there are some circumstances when you're waiting for an important call and you have to check every incoming call while you're on the other line to see if it's the important person, but I feel like there are some people that answer all their calls without sending any to VM as a standard practice. It's annoying.