Different boats, I agree. Skype is an additional plugin I think, so you don't need to have skype installed on your device. But that's the whole point. Google is the new Microsoft, though probably not as much evil. They have wide interests in almost all areas from search engine, android, chrome os, social, browser etc. So they can really control and push what fits their needs.
Tomorrow if Facebook comes out with their own browser, they would surely push skype baked in, into the browser rather than make it an additional installation.
When I mentioned the word "monstrous", I meant that in terms of the implementation and code size. I am not sure of XMPP, but SIP I think is. There are tons of new rfcs, specs and drafts that have come up which one has to implement for effective interoperable SIP sessions.
> Not sure what SIP or XMPP are "monstrous" though.
Both SIP and XMPP are way beyond "monstrous", hundreds of specs, and most implementations are not really interoperable beyond the very basics. (XMPP is still barely at the level irc reached almost 20 years ago, and that is after ten years of and ever growing and changing list of specs, which is actually the problem).
Packing the necessary tools into a toolkit that a browser can utilize first-rate is, as you pointed out, a different challenge. Not sure what SIP or XMPP are "monstrous" though.