Most likely because you had a previous version of Outlook installed. Unlike
other Office applications, there can only be one version of Outlook
installed at a time.
You must first uninstall any previous version of Outlook indeed. Then run
the setup of the trial again and do a custom installation to verify that
Outlook indeed will be installed.
And if you remove the none functioning previous version of Outlook without
determening the cause, then the new install may also produce the same
problem
If you are sure you downloaded Standard and not one of the versions without
Outlook, rerun the setup (modify/add) and make sure you choose Outlook to be
installed.
You could check first to see if the Outlook .exe file in the Office
directory with the Word, Excel, etc. .exe files.