I guess it is a bug in the scroll down trigger list display. I am right now creating a flow and when I type in the search box the word "Office", four different options are displayed. However, if I scroll down without filling the search box, these 4 options are not displayed.
@Nicovddusse when you create a flow, the most popular triggers are provided in the trigger dropdown. When you type something in the textbox with the prompt "How would you like to start?", a search is performed with the user input and matching triggers are returned.
As @efialtes mentions, there indeed are 4 triggers available for Office 365 Outlook. Infact for almost all of the services provided by Microsoft Flow, there would be atleast one trigger available - not a hard rule but a practice that we try to follow.