Manners. It's just rude if I'm not somewhat close to the person.
I don't always _feel_ autistic, but stuff like this reminds me that I'm not normal.
Being too flowery and indirect is annoying but not impolite. If you overdo it then people may still get annoyed with you, but for different reasons. For most situations you don’t need too much, a salutation and a “I hope you’re doing well” and a brief mention of who you are and what you’re writing about can suffice.