That’s because EGroupware itself was traditionally a mail client and not a mail server. You can connect it to any mail server, but it has the ability to administrate a mail server.
EGroupware Hosting offers a mail server integrated with EGroupware. As does the relative new EGroupware Mail package for on premise installations.
As the mail(-server) part is optional you can access it either with a right-click on your folder tree and select “Mail account” or as an administrator by right-click on any user in the administration and select again “Mail account”.
In EGroupware Hosting we’re hiding IMAP and SMTP tabs, so you can not break the integration (by setting wrong values).