MMOs don't handle endless player progress well, even if it's a secondary leveling system. Players also get very upset with systems where their progress gets reset and they feel like the game is taking things away from them that they had earned.

Level caps serve to give new players time to catch up to veteran players. Any sort of secondary leveling system that's only available after achieving level cap would effectively become the new level cap and be seen as mandatory by players, especially if rewards offered any sort of game advantage no matter how minor. It would become another tool players use to discriminate against others.

As for rewards that don't offer a game advantage, we've already got plenty of those to farm. They serve just fine as goals to shoot for once we reach level cap. No need to create a secondary leveling system to give those rewards when they can be added to existing content or other content planned for release.

Honestly, it's better to have a hard stop for progress than an endless system that makes players feel like they aren't allowed a break from the game. That leads to stress and burn out.