Because the different transports have vastly different abilities and limitations that are a source of frustration if you attempt to obfuscate the differences between them and aren't served by a unified UI?
I look at iMessage and I think it works pretty perfectly, SMS when no internet or to a non apple user, iMessage whenever possible. What I want is for whatsapp (and I suppose facebook and plausibly email) to do the same all in my single messenger application.
But. Ignoring the need to get the tech owners to consent to this new "trillian", I'd also have to trust the app itself. Good luck making money on that.