I've seen that in many makes so far. There is a reason for this and in case you missed it I'll duplicate a short answer for you.
The reason the output is a fixed volume was that if it was variable then a lower setting may be connected to an amplifier and the owner would complain about the hiss and noise. By fixing the output level this solves that complaint but leads to your issue.
Complaints are not good, limitations are acceptable.
I am outputting audio from analogue outputs to my hifi amp to get better sound which is a marked improvement, however the output level seems fixed so that I have to adjust volume on the amp. Is there a setting in the TV system menu which will enable the analogue outputs to be adjusted by using the TV remote?