Wahaj,
Thanks for the screenshots. They prompted me to investigate further and I found something interesting; if I create from the legacy customizations experience (as your screenshots depict) the activity does get added and is visible and selectable anywhere Activities show up. If I create from the maker portal as a new table, it shows up in the option set (Choices>Activity Type) but does NOT show up in the UI (ex. when I go to Sales App>My Work>Activities>Activity Type drop down).
So, that's an interesting learning for me!
Katie, apparently all you need to do to make sure it shows up is create in the legacy experience and you're good to go. My apologies for causing confusion for you.