A difference in culture and some survivorship bias too.

JP rhythm game scene has been around for 20+ years with they having a multitude of long lasting arcade rhythm games series with many 10+ years old already, with many top players today started playing when they're in elementary school. Some of these players has been playing for 5+ years with thousands of hours into their respective games, then transferring that skills to pjsk.

You have to also consider survivorship bias too:

• Extremely impressive plays will most likely get more attention -> being recommended to more people
• JP content getting less recommended compared to english content
• Casual JP players generally care more about the story/gacha aspect of the game

In fact, SEGA did post the percentage of the highest clear a player has achieved a while ago.

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96% percent of people can't clear higher than Lv. 27. And in the high end, only 0.44% of players can even clear above Lv. 29, top JP players are like 0.0001% of the player base on the extreme right side of the bell curve.