First, Ezreal's ult is the same (Actually most abilities are the same, it's an extremely common ability type). Second, you're asking them to change the entire ability's type and shift it over to a completely different one just because it looks similar to the other type visually. Lux's ult can't be interrupted because it has already finished casting by the time anyone can do anything about it, it's like asking why you can't CC Garen out of any of his abilities.
Just because an ability has a forced animation doesn't mean it's automatically a channeled ability. Why should they change one ability that acts consistently with other abilities of its type? Plus, if they were to shift Lux's ult into being a channeled ability it'd mean that they'd need to rebalance it to fit within the channeled ability power budget (which is much higher than its current power budget, by the way).
Also the counterplay in these telegraphed animations is so that you can avoid it or kill the caster. There are very few times where someone is in range to interrupt an ability while also not being able to simply avoid it (or kill the caster) in the first place.