so why the devs never use 90% of the content for end game?

[Hasrat]

why not udgust them for the cornet end game lvl and make the tome grind a more fun thing where you don't just run the same 3 dungeons for half a year.

Because this amounts to "Just do more work." Adjusting old content to scale isn't something that just gets done overnight. The work required to accomplish this would 1) mean that you're sacrificing new content for scaled old content, and 2) have to be re-adjusted every patch to continue to scale with growing stats.