Very good point. I do not believe that a plane can just disappear, and that they are without any effective tracking device.
The aircraft didn’t “just disappear.” It encountered severe weather. The pilots requested a course change to avoid the weather and started climbing. Shortly thereafter, Jakarta (Indonesia) ATC lost contact with the aircraft.
So, primary radar - if it was being monitored by Jakarta - failed to detect the aircraft, which means the aircraft did not reflect RF energy, and secondary radar did not receive a response from the plane’s transponder.
Note: I am not a pilot. I do not play one on TV. And I did not stay at a Holiday Inn Express last night.