Judging by my contact list and the lists of others I see, statuses and presence are a really big deal: people use red-dot to mean "I'm here but interrupt me only if it's crucial" and green-dot to say "fire at will!".
Hangouts supports none of this, only a very simplistic "I'm actively on a device". There's no way to append "... and I'm working, so please don't interrupt unless you have to". You can mute notifications, for a preset amount of time, but that's not the same either -- it's turning off your phone rather than setting it to silent.
I guess they see this as "advanced" functionality, but judging by the people I know it isn't. They may have just copied Facebook, but Facebook messenger isn't something people leave on all day, and it also allows you to selectively block groups of chatty people until you're ready for them.