Probably because they're mobile and disconnected most if the time, so they don't have to advertise your availability status periodically. In the 2000s they mostly advertised that the user was away from the terminal. Nowadays one has the terminal in their pocket. Some of the old IM systems have a mobile status now. The busy status was ignored successfully then as it is now.
The busy status, at least in some clients affected what sort of notification the recipient got. My phone does have a global do-not-disturb function, but it doesn't notify the sender, and the only real bypass for it is a voice call.
I want to be able to pre-notify all senders that I won't be replying quickly so there's no confusion. In an ideal world, I could set this in one place on my phone or desktop and the 8 different chat apps I use would all get respect it. Of course SMS can't support it, but nobody who has anything important to say to me would send me an SMS.
