Why Apple Reminder app on iPhone 7 / iOS 13.3.1 does not have hourly repeat option?
Had the same problem with an iPhone 11 Pro, so it's definitely not the age of the phone.  Could reliably follow the instructions from this answer on an older Xs, but Hourly would just refuse to show up on the 11 Pro.
Then I noticed that the summary page on the 11 Pro (tap &lt; Lists in upper-left from any list) had a link Upgrade right around the heading My Lists (sorry, didn't think of taking a snapshot), and tapping it asked for a confirmation (the upgrade is irreversible &amp; breaks compatibility with other devices sharing the same lists using older software--see this Apple Support page), and then voila!, Hourly magically appears in the list of base periods on the Repeat screen.