it's not. An economy where only a select few benefit from the GDP (e.g. via stocks - the richest 10% of Americans own 93% of the stocks!) is not a "quality of life" measure at all.
It's not a good one though, because weird effects like the AI bubble incest investment web artificially blow up the GDP, and because it doesn't reflect the economy "feeling" the population experiences.
To expand on the latter point - say you have automation enabling more economic growth. A significant amount of people lose their jobs, others are afraid they'll be the next ones on the chopping block, and people hold their money together as a result - if you ask general people on the street or in representative surveys, you'll get the feedback that the economy is going to the dogs, but "the numbers" don't reflect that.