You’re not missing anything, as the Power BI REST API only allows pulling metadata but does not allow extracting data rendered in visualizations. As of now, there is no REST API endpoint available that outputs data in report visuals (such as tables or charts), even with service principal authentication.
Below are some suggestions that might help you:
Use Export to File API to export a report or specific visual to a PDF or PowerPoint.
If your primary concern is tabular data, embed the report into a workspace using Power BI’s XMLA Endpoint. This enables connecting tools such as SSMS for querying semantic models (datasets) utilizing DAX and MDX.
Alternatively, DAX queries through XMLA endpoints can be employed to obtain actual data behind the visuals you need—often this is a more reliable workaround.
For working with multiple sources and reports in Power BI, data integration platforms like Fivertran, AirByte, or Windsor.ai streamline and scale these processes, aiding faster data extractions and automating reporting.