If parsing is needed to clear challenging content then why is it not in the game?

[GMERC]

From a game design point of view, if the game has DPS and HPS challenges, the player should be provided the tools to measure their performance against those checks in my opinion.

You already have that. If you fail a DPS check something happens and everyone knows it. It's not some mystery that waits until you're at 1% on a boss before randomly killing everyone. Enrage timers aren't some profound and mystifying concept, either. A parser is useful for seeing weaknesses in your group and in yourself, but it's far from actually necessary to accomplish things.

Parsing just gives you more information, and it doesn't need to be built into the client for you to take advantage of that information.