Because for admins, while your reasoning makes sense, the main issue is probably trust. Adding an admin to your instance is basically giving them the keys on everything, and allowing them to destroy the whole thing.
I know instance admins tend to be really reliable, but I also understand why some might be reluctant to add 4 different admins (let's say 2 more per time regions, and assuming you need 2 additional time regions to cover 24/24) to their instances.
Regarding your edit, the user behind the account took a picture from Hexbear. I saw something else yesterday on another spam account that it was from /qa/ whatever that means. 4chan?
To be fair, it might be nightime for the moderation/admin team of that instance. I know for instance that for our instance (Jlai.lu), if that would happen during nighttime in Europe, we would probably not react within the hour.
But wait, I had a similar issue:
Is this normal? It seems to contradict your experience
Also, if that may help, I don't see the spammer messages on the post in !science@mander.xyz