com_content frontend form model extends the backend model which in turn extends Joomla\CMS\MVC\Model\AdminModel. The backend model has a custom save() method where the event is triggered.
Meanwhile, your frontend form model extends Joomla\CMS\MVC\Model\FormModel which has no such event and no save() method. The simplest solution would be to trigger the ...
All of my demonstrations will assume this line is already declared:
$db = JFactory::getDBO();
I will also provide a "guinea pig" table as a demo reference and playground for researchers when sample data is necessary for context. (db-fiddle.com demo)
CREATE TABLE IF NOT EXISTS `#__calendar` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`month` varchar(100) ...