If school lockers in reality are actually slowly becoming less common over the years across schools, why do a lot of media still sensationalized them as a symbolic part of school life?
Because reality is unrealistic. Television shows are written by people who had lockers (and often for people who had lockers) and need to be sold to network executives who had lockers. Also, the schools where most shows are filmed are in places that still have lockers. And thus we get schools portrayed as being rife with lockers. Though I’m not sure how inaccurate this really is. I graduated from high school less than 20 years ago, and the building still has just as many lockers as when I was attending. That’s just one data point, of course. But it’s not like school these days are universally bereft of lockers in the hallways. The transition is as yet incomplete.