Why market value gets so equal?
because it's the architecture of the game that way.
I suppose the first developers weren't very optimistic and didn't expect that the game would be so successful and players would continue playing after 1,5 year
It's not only the values, more things stop after some time.
At lv (about) 20, you finished with the stadium/facilities and almost all the achievements.

I remember back to 90s I was playing a game in ps (yes, 1 ), XENOGEARS, of Square/Enix of Final Fantasy series.
It was a rpg, two CDs.
Amazing, the story, battle system, items/gears, world map, places, transformations and transportation, heroes, music, emotions.
When I put part II, the 2nd CD,most of it was narration and battles till the last one vs the final boss.

Later I read that at some point, the budget of the project almost finished, so they complete the game with an not equal second part