It’s because the option to switch back from Gmail to Googlemail is only on the ‘Accounts and Import’ tab, not the ‘Accounts’ tab that is shown in the screen image. The ‘Accounts’ tab shown in the image is part of the older version Gmail user interface. If you click the ‘Newer Version’ hyperlink in the top right hand corner, then Gmail will show the ‘Accounts and Import’ tab in Settings and you’ll then be able to switch from Gmail to Googlemail (assuming that you had a Googlemail address in the first place).
Edit: Now that Tom has updated the original screen image to reveal the top of the window, it is clear that Gmail was running in Basic HTML View (as apposed to Standard View). Running in Basic HTML View is another reason why the ‘Accounts and Import’ tab doesn’t appear and therefore why the option to switch between @gmail.com address and @googlemail.com address doesn’t appear.
If you are running Gmail in Basic HTML View then you need to click the ‘Standard View’ hyperlink in the top right hand corner.
If you are running Gmail with the older version Gmail user interface then you need to click the ‘Newer Version’ hyperlink in the top right hand corner.
Having done both of those things if necessary, then the ‘Accounts and Import’ tab should be available within Settings to allow the option to switch between the @gmail.com address and the @googlemail.com address.