Email service providers keep the username active for 180 days without having to check the inbox. After 180 days, your id will be declared dormant and you have to activate it back. In case of ID not activated for 3 years, the username will be made available again.
This is how the Yahoo and Outlook.com policies are made for the free email address.
In case of gmail it can be activated simply by opening it just once. And considering the fact that gmail ID mines personal data, it does not delete the data. And with gmail your data remains with them forever. You do get chance to recover password using 2 step authentication. So they don't have to delete your data and account.