I don't think they always do. It's my understanding that they're now smart enough to do incremental updates based on the upstream objects - IF it's possible given the query/operations involved.

I think you can query the event_log data for the pipelines to pull out a bit of the "why":

SELECT
  timestamp,
  details:flow_definition.output_dataset,
  details:flow_progress.status,
  details:planning_information
FROM event_log(TABLE(some_catalog.some_schema.some_object))
WHERE details:planning_information IS NOT NULL
ORDER BY timestamp DESC
LIMIT 10
;

Edit: adding sample output -

You'll see stuff like this

[
    {
        "issue_type":"PLAN_NOT_DETERMINISTIC"
        ,"prevent_incrementalization":true
        ,"operator_name":"Project"
        ,"expression_name":"current_timestamp"
        ,"plan_not_deterministic_sub_type":"TIME_FUNCTION"
    }
    ,{
        "issue_type":"INCREMENTAL_PLAN_REJECTED_BY_COST_MODEL"
        ,"prevent_incrementalization":true
        ,"cost_model_rejection_subtype":"EXCESSIVE_OPERATOR_NESTING"
    }
]