How to completely "reset" an Oracle instance?
There are just so many things in the database in addition to user data that checking all of them would take a considerable amount of time. Also there are some parts that were not meant to be removed at all, for example the history of used features (for license audit).

Sure, you could try to check all non-default configuration or rerun catalog scripts (but whats the point of that? creating a new database does exactly that). Its easier to just drop and recreate in 15-30 minutes depending on your selected components.

PS: there is no reinstall involved in dropping and recreating a database.