Question about Lux Ult (not a QQ, plox nerf lux. Legitimate question about design)

I've been playing league on and off for a long time, and only really started trying out ranked this past year. Something that's been bugging me for a while is a little facet of lux ult. Either it can't be interrupted, or the timing for the interruption is very small. Now, before I continue, let me say that I'm not asking for this to be changed. I'm more curious as to why this choice was made. For such a high damage large area ability, to be uninterruptible is a bit odd to me. Same with Ezreal ult with the same mechanic. These two are the only two spells in the game that appear to have a charge up time but in reality have no way of interrupting them aside from killing the champion casting it. This seems like it would be rather limiting, since in the case of a teamfight where a teammate is locked down you would want to be able to stun the enemy lux/ezreal to stop them. Now, i know you can dodge these ults, and thats why I'm not saying this needs to be changed, I'm more just curious as to the reasoning behind it.