Instant Messaging on Linux

One important area where Linux is significantly handicapped is this. A decent IM client which allows voice, text, video streams is missing.
Shame on yahoo and ICQ (MSN can be forgiven for overlooking Linux, and google only has text which can be accessed just as well with the web browser).
Kopete the IM client in KDE seems to be the best option. It supports multiprotocol IM, with yahoo, msn, ICQ and a few other protocols being supported. The support though is rudimentary with many features of the native clients missing (as is also the case with Windows multiprotocol clients like Trillian).
The question that needs to be asked is what prevents major IM services like yahoo and icq from offering a full blown linux client. After all they make money by displaying ads and the more the clients used the greater their earning potential.