What you have noticed is correct...

"fields_view_get", is method which will be called only once, that is to load the view, as the name indicates, it will load the XML view & the fields associated to that object...

once the view is loaded, then the datas are fetched from database, after that the view is unaltered...

So when you open a form, only once the function will be evoked... So does in the case of tree view, however since you click on the action menu, it is like evoking the action newly, inturn it will evoke the "field_view_get" again, seeming it look like it is called repeatedly, but technically it is no, ...

you can get active_id in fields_view_get in odoo 15 using self.env.context.get('params')['id']:

Example: 

if self.env.context.get('params') and 'id' in self.env.context.get('params'):
    active_id = self.env.context.get('params')['id']

Thank you for the response. However, I have also observed that when I select the 'Tree' view from the available modes on the right side, the fields_view_get gets invoked each time. But it does not happen  in the case of form view. Also, do you know how to get the context.get('active_ids') to work properly?

Don't expect context.get('active_ids') to be populated properly in fields_view_get.  That method is invoked BEFORE any search/read on the selected model is carried out.  If you can described what you are trying to achieve, maybe we can help you better.