Why does trist have a jump that breaks cc AND resets

actually it does break CC

If your CC lands:

Before she casts: She can't cast Rocket Jump due to being CC'd. Mid-Cast: Rocket Jump Resolves and she's CC'd at her landing point. Mid-Dash: Unless the crowd control has higher priority than the dash (i.e.: displacements, taunts and suppresses), Tristana will be CC'd at the end of her dash.

She'll get CC'd, but a large portion will be "wasted".

It makes no sense why some dashes can break CC, while others are 100% completely interrupt able midway/quarter-way/whole-way/beforehand.

That's how it works for all CC skills, but the impact is not the same on every champion,.i.e.: being stunned on top of her target doesn't give Diana a reset, and that's probably worse than Lunar Rush not going through.

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actually it does break CC, there is an un-interrupt able cast time, to both her jump and Ezreal blink. Both of which, if timed right will ignore hard CC, like a blits Q or thresh Q, and put off other hard CCs such as snares/immobilizers until after the jump has taken place, even if you land it before they move. Which actually reduces the amount of time they are snared, as the timer starts ticking as soon as you hit them, but then they jump/blink and the whole time they are moving the time is still ticking down, yes its a minuscule amount of time, like .25 secs for tris less for ez, but still none the less it does reduce the actual time snared.

This is what is known as a "casting timer" and is almost universal to all non-channeled spells. There are a handful of non-channeled spells without a casting time, but I don't think you want Rocket Jump or Arcane Shift to have no delay at all.

ALL hard CC should cancel ALL dashes/blinks, as well as Snares/roots/immobilizers that hit before or during dashes/blinks. It makes no sense why some dashes can break CC, while others are 100% completely interrupt able midway/quarter-way/whole-way/beforehand.

I can't think of a single dash that is actually interrupted by any non-Airborne type CC. All of them are capable of completing the dash/jump/blink before stopping, so long as it's cast before the CC begins.