Yes I agree, it's weird. I think it's because developers are focusing more on mobile applications as almost everyone is doing everything from their phone these days, shopping, paying bills, social media, even doctor appointments.I think it's kind of weird that no one has made something like Skype, or even ICQ, for that matter. Why does everything have to be so damn complicated these days with apps, phone numbers, and tons of personal info?
Session is pretty good, but in my opinion, it’s missing a decent UI.
There are things like XMPP with OMEMO which is actually very secure and works across all platforms. However it can be a bit tricky for normies to get it setup. Apps like Signal is far easier and a plug and play solution. The majority of people are lazy, they just want it to work without doing anything.
But yeah a old school style messenger like ICQ with E2EE would indeed be awesome.