I suspect that's because once you reach the end-game, all cars end up with the same, 5-star parts and at that point it stops being about the tuning and starts being about the actual driving. And if the racing mechanic isn't good then there's nothing left to do but start the grind over.
Motor City Online was great, even if it never delivered what it promised. That was the first instanced MMOG and people didn't get it. If they hadn't advertised it as an MMO it would probably have done better, or if they'd used the Porsche engine rather than the previous game's. People wanted to see hundreds of cars shooting around but they couldn't do it on NFS4 tech. But the actual racing and modding bits were great.
The fact that it was all dyno-based meant that there was no "best part" so each car remained different. And the fact it was all classic muscle cars just made it cool
I'm sure it could be done now though. A working city to drive round like Burnout Paradise, the range of parts and events that MCO had, police chases like Hot Pursuit and a physics engine like Porsche Unleashed. That spells win.