Because flight attendants are dealing with high double digits (probably low triple digits if it is a flight with meal service) and don’t have time for individual passengers’ bullshit.
The time it takes to figure out who in a given row to ask and then the time to nudge them awake adds up REALLY fast and leads to the people in the back (well, middle) of the plane getting severely delayed service.
If you want a wake up call? Ask the person next to you to nudge you awake if you are asleep when meal service starts. I am 100% the kind of person who pops his headphones on and grabs a kindle or a steam deck the moment my row is seated. I have zero issues with someone asking me to nudge them when meal service starts.