At a certain big airline in question there is no de-brief after a flight, no going back to any crew rooms, and no seeing any managers. We just get off the aircraft and go straight home.
The PA is the final time of the entire trip where the in-charge has the whole crews attention. 95% of the in-charge crew members at said airline thank the crew via the PA. The 'thankyou' PA is more noteworthy by its absence in the rare event it is not heard!