As someone who used to use multi-IM clients (Pidgin, Trillian, Meebo) back in the MSN/AIM/ICQ days, I have to say, it's surreal to see how the conversation has shifted to the point that this is now a reasonable-sounding comment.
Maybe they wanted to, but it was too onerous back then to force client software updates that they had to continue supporting reverse-engineered versions of the protocol?
MSN Messenger and Yahoo IM both cried foul about this and called out for interoperability, but then turned around and denied third-party clients access to their own networks once they got big enough.
I bet if HDMI and USB wouldn't exist, they'd protest against being able to connect their MacBooks to non-Apple TVs and non-Apple keyboards - to keep them safe from evil Samsung monitors and bad Logitech mice.