$$$$ is the big reason.
LCD needs it's own full ticket share.
LCD doesn't help AF sell out arenas, AF can do that with normal openers. Prices might go up, but that's dangerous to do when you're trying to sell as many tickets (very curious to see what Arcade will be charging this tour... I have a feeling it'll be sticker-shock-y)
but yeah, I see where LCD is in a weird position, like, I don;'t think they could play an Arena show in LA. Hollywood Bowl maybe.
In the rest of the country that isn't a huge city, 5-7k feels right, but that's a weird venue size.