Why doesn't xerath ult get a slow but jhins does?

You can't body block a Xerath ult :P

Xerath ult range is longer, can fire in a full circle around him instead of 1 direction and does AOE damage instead of single target.

Because Xerath's ult has an insane range and isn't awful in terms of damage early on. Nor does it have a limitor in terms of width.

Why doesn't Darius ult stun like Sejuani's ult?

Why doesn't Garen get four shots of his ultimate like Irelia does?

Why doesn't Xerath have AS boost his AD instead like Jhin does?

It's almost like they're two different champions that function differently.

Xerath ulti is by far one of the worst ultimates in the game. But Jhin's ultimate is more what xerath's ultimate should be like, not the other way around.

Because you can always just buy Rilays to make them slowed... Jhin gains no benefits from buying Rilays unless going full AP.