Because the object cache caches query results from the database and if you modify the database – either directly or using WP’s DB APIs – you to clear it. Otherwise you’ll end up with inconsistent results.
Using WP-CLI is probably easiest for this: https://developer.wordpress.org/cli/commands/cache/flush/
If you don’t know enough to write these PHP commands yourself, using some shell scripting and the BuddyPress WP-CLI commands will probably be easier: https://github.com/buddypress/wp-cli-buddypress/
