Select All Should Be Included. if other email provider's can included this feature, then why not outlook?
Derek R commented
There is already a select all button. If you're in a folder, look at the top of the email list. To the left of the name of the folder is a check mark inside a circle. If you click that, it will select all emails in the folder.
Alternatively, you can also use the Ctrl+A keyboard shortcut instead.