The first is that spammers automate so if you don't then they're always much faster. By the time your humans are paying attention and have made a decision it's too late, all the spam (scams, frauds, malware sites...) was already delivered. In the next spam run everything will have changed, so, decisions made by that point are useless.

The second is that your suggestion contains an unstated premise that the human evaluators would have somehow more information to work with than an automated system, or would reach a different decision. In reality they don't and wouldn't. URL reputation systems like this are triggered by spam attacks. For a certain window of time there's a high probability that any message containing a specific domain name will be flagged by users as spam, so the system short-circuits that and starts classifying all messages of unknown status containing a link to that domain as spam. This works well because spammers usually want their targets to visit a website.

So the human evaluators in this case will see a message like:

"URL domain signal.me has 67% chance of spam and rising, confirm block? Y/N"

and the humans will always press Y because obviously (a) it means that such a decision is right more often than it's wrong and (b) the domain name is normally meaningless anyway. The block will be removed a bit later once the spammers go away.

In this case, it's tempting to think that some human in a cheap labor country would somehow see this message and think "ah! signal.me! clearly a domain linked to the super cool Signal messenger, which I personally like, so I won't block it even though this might cause a lot of people to be victimized by criminals". But they wouldn't and shouldn't. The domain even looks phishy, it's quite surprising to learn that it's a real Signal linked domain.