The level 9 event-horizon

I have been in many campaigns now over the past 10 years, and one thing I consistently noticed is what I define as the "level 9 event-horizon".

Level 9 event-horizon: The moment the party levels up to 9, something fundamentally breaks in the game balance in the party's favor that rapidly keeps getting worse.

At lvl9, the party reaches a point where they can beat pretty powerful monsters, like even CR12-13. This means they can basically resolve any "normal" combat quest people would give to them. This means rewards, quite often magic items, if the world has them (which most do). More money and magic items mean more resources and raw strength that allows them to take on even harder stuff.Eventually it gets to the point where even though they are still lvl9, they need freaking adult dragons to challenge them, and they still manage to beat those with appropriate preparation. Adult dragons have hoards, with magic items in them, so it keeps getting worse. In one campaign, the 5-member lvl9 party started making preparations on how to beat a Demogorgon. Like how is a CR26 monster even on the table for a lvl9 party? And the worst is, I think we had a decent chance of success, although the DM never gave us the fight.

I don't know what causes it to happen at lvl9. The lvl5 spells? The proficiency up? Simply the raw power of the party just reaching a critical mass? I don't understand, but I noticed the lvl9 phenomenon in many campaigns. I pointed this out to many DMs, saying "maybe they should give less magic items?", but the answer was always the same: "the party just beaten a really difficult challenge, so they need to be rewarded appropriately".