There are layers to it having a lot to do with social class, location, and a player's height. This isn't a knock on schools with sattelite campuses like San Beda and FEU or schools with an unfortunate stereotype of pang-basketball lang like St. Francis of Assisi, because there's a lot of those all over the country... but the reason why parents in southern Manila who can afford to send their kids to Zobel do so is that they know it looks better on their kids' application form for DLSU and CSB while being known to produce students smart enough to be admitted to UP and AdMU on their own merit. That said, it's only about max-150 boys per batch in the 90's at Zobel who were at least 5'6. If a kid is taller than 6'4 before their growth spurt and can really play, Badolato or Jarin would have surely gotten a hold of them first for San Beda or Ateneo... so literally all Metro Manila high school teams in the 90's had like 6'2 centers and 5'9 point guards. Ultimately, Zobel wasn't really special in that regard... they just had better "bigs", bigger Jason Webb-type of guards, and definitely higher-level coaching than Muntinlupa National, Southridge, and San Beda-Alabang.
It's only 18 high schools teams that are affiliated with a college hoops program and a handful of them will surely scour the Earth for Filipino blood and talent. Then you factor in players from other leagues like Tiong Lian and CESAFI Juniors looking to impress college coaches. If Palarong Pambansa is a pipe dream instead of a formality, then you got NBTC (where that fight happened) and SBP-Pasarelle where the latak of the bunch have the most intense, dayo-like games.
