A spammer from a given IP address creates an account, sends spam, has their account destroyed, and repeats until their IP is blocked from posting.
At this point, if they again create another account from that IP, there is nothing productive they can do with that account—either for themselves as spammers, or for SE (if they had a miraculous change of heart and decided to try being honest participants in the site) ... they're blocked from posting.
So then why let them create accounts at all?
There are, for sure, specific IP addresses that are bad actors. Over the past few months, each origin has created dozens of accounts with obviously spammy names, about one every 2 to 3 days. They're SpamRammed so they cannot post, but they set up profiles advertising dodgy sites. We notice and destroy them, as we try to catch spammer accounts before they have a chance to spread their noise; but they pop right back up within a couple of days, from the exact same IP as before.
Since these accounts can't do anything useful even if they were so inclined (which, clearly, they're not), there's no point in allowing them to be created in the first place. This would prevent them from just taking up space, or from waiting for SpamRam to forget about them, and then posting their spam.
So ... can new account creation from a spam-blocked IP be rejected?
Edited to add:
In this answer to Why are suggested edits that were rejected as spam or vandalism not visible to users who don't have an account on the site?, we see @Oded note that
The theory is that spammers have clients - and that they need to show them the spam [....]
If they can't show the spam to their clients... less incentive to post [...] spam.
Right now, new accounts ip-blocked from posting can still have spamvertisements for profiles, which allow spammers to show the spam to their clients - and this is indeed likely to be the only reason they keep making these new accounts. If the platform includes measures specifically to prevent spammers benefiting from suggested edits, why not prevent them from benefiting from account creation at all?