Why wasn't Renekton/Xerath given the Darkin treatment of sealing?

Because they were both entombed before the Darkin war even began, only breaking free ages later when Sivir and Cassiopeia came to Shurima? They wouldn't have any reason to be sealed into weapons since they didn't participate in the Great Darkin War.

Cause they weren't deemed as Darkin, also they were both locked away within a tomb before all this stuff went down iirc.

Even better question, why didn't Targon stop Fall of Shurima, essentially Xerath?

Shurima wouldn't fall, Runeterra would still have their protectors under a ruler, another potential danger for Runeterra (Xerath) wouldn't exist.

It is clearly shown that the Great Darkin war lasted for generations to the point that ppl even created a new word to call them.

Xerath and Renekton may have fought for days, but the Twilight of the Gods happened centuries (if not millenia) after.