Because AFAICT by the example, what is desired to be expressed is Alex is associated with gleasonator.com and also associated with mostr.pub. Which to me means that Alex could wish to have two different NIP-05 addresses. Which isn't currently a supported thing.
Discussion
alex@gleasonator.com@mostr.pub -> look up pubkey at https://mostr.pub/.well-known/nostr.json?name=alex@gleasonator.com
Alex IS associated with both domains, but there is only one NIP-05 identity here. Is there really an issue with having more @ signs? If he wanted to go by `@lex_gle@son@nostr.com` would it be any harder to look up than `alex_gleason@nostr.com`?
Of course I don't care about giving Alex his leet hacker NIP-05. I am wondering how we might proxy NIP-05 identities like Alex did for events with NIP-48. Allowing for @ signs would be unconventional, but seems like the simplest way to do it. Whether it is reasonable is another question.