Why is lightning jagged instead of smooth?

Chaos.

“The propagation of the lightning channel follows a path of the least resistance, which is not a straight but a zig-zag line because impurity or moisture in the atmosphere causes the air to be ionized in different directions. Sometimes the lightning channel finds more than one conductive path and branches out, giving lightning its forked appearance.”

Source

And curved lines are not the path of least resistance

It kinda does

I’ve seen lightning curvy, ball lightning, and snow lightning, which is just a momentary shapeless glow, and the thunder says, “poof”.

Some crazy strikes

Ball lightning

Snow lightning

I’ve also seen this but it is much more impressive seen first hand and up close.