In a practical sense for the frequent traveller you're probably right, but the reason these briefs are done on each and every flight is due to airline liability if things go wrong.
Even straight forward things, to most people, such as keeping your seatbelt loosely fastened at all times, if not announced, leaves the airline open for claims if someone sustains an injury due turbulence.
If the airline has briefed the pax on the safety requirements then it's up to the individual if they want to take notice.
There are always first time flyers on every flight who need to be properly briefed.
Regards,
BH.