I would not say it is weird: there is only one Facebook and most people only sign up to it once.

There are many email address providers and many people have many email addresses.

Many people have their email address provided by their employer. And many people use this address as their contact address in even non-professional capacities. I presume such people also use their work address to sign up to web sites/services. When they change jobs the old address dies.

The same applies for students in schools or universities and it once applied to the ISP's customers too.