Maybe in US this holds, but things may be different in different places. I'm a Chinese living in the bay area. I think people here treat E-mails as part of their daily life probably because most of them began to use it before the mobile internet is a thing. On the contrary, in China where the majority internet users are mobile-savvy, WeChat is the new email. Actually E-mails are only for official communications there…
The majority of Internet users in the US - nearly 80% of US adults have a smartphone - are mobile savvy and were that way before the Chinese majority got there. Being mobile savvy has nothing to do with it.