"Because just like a drug addict, they can't help themselves. They think that the ball is going to make them a better bowler, then they find out their game still has flaws, so they go out and buy another ball."
My good friend 'The Curmudgeon' is content with averaging 190-200 while playing cards between shots, so I wouldn't expect him to appreciate the value of a larger arsenal. Different strokes for different bowlers.
For myself, it's not about trying to find magic for a league shot. It's about having a broad variety of equipment (different cover, drill, core combinations) that can be used on more challenging conditions. Contrary to what some believe, it's not about having an old Urethane Hammer, change your hand position, and you should automatically score on anything. The modern game is about equipment matchup in addition to prerequisite skill. You generally have to be a bit of a ball whore to get there.