Why does Lux ult still go off when she is stunned?

It's not a channel.

This. I ashe'd a Lux and still got decimated by her ult lol

Because Demacia.

Because you cant just negate someone's entire ult just because you stunned them, it's why they stopped qss from removing zed ult.

does too

Because its cast time is shorter than its animation time. Once the cast time has passed, the spell will go off, even if the animation has not yet completed. If you interrupt Lux during the cast time, it will stop the channel. However, the cast time is much shorter than the animation, so it feels like you should interrupt her, but you really just got there after the cast time completed.

how do you catch light?

both ezreal and lux ult will channel even if you hit them with a cc mid cast. i once casted my ult got eaten by tahm kench it shot the projectile and when i came out of kench i did the animation lol.

Animations.

It's not a channel, just a really long cast time (there is a difference).

Lux's ultimate isn't a channel, it just has a longer than average cast time.

CC doesn't interrupt cast times.