Of course they don't need email, they can all see each other! Try working in a company where projects routinely span a handful of countries.

Most email isn't personal anyway, it's often machine generated by the various tools we use, reports of bugs changing status, servers coming online, servers going offline, jobs finishing etc etc.

email is actually there for a reason, it's just just there to interrupt you. At work you are expected to maintain contact with those you work with and email is one of the ways we achieve this across time zones and geographic boundaries.