I can think of a few reasons.
First of all, it is possible that someone is trying to ruin your IP reputation. So if "someone" subscribes to your newsletter, you should request verification of the request by sending a link with a unique token. Do not send the newsletter until the subscriber has confirmed their intention.
If you start sending your newsletter prior to securing approval, then the victims will complain.
Sending a confirmation link to someone who turns out not to have requested anything is inconvenience for sure, but at least the game stops here.
The "attack" may be distributed over many different IP addresses, but try to have some common sense defenses in place ie: do not allow multiple signups from a single IP address within a short time frame. Even though I hate this, you may have to implement a captcha to thwart spamming attempts.
You can also decide to review all signups manually, on a daily/weekly basis depending on your schedule. Keep the new subscribers in a queue and discard the records that are obviously not genuine, like made up surnames. Any database requires maintenance and quality control, so the best is to control data quality ahead. You have a data quality problem, so it's time for the human being to take over and monitor the machine.
On my websites, I have an admin panel from which I can quickly "unapprove" new subscribers, it's just a matter of ticking checkboxes. If you don't have such tools, then try to write at least a small SQL script to review your table and make the task less tedious.
But the real motive could be more basic than you think. Spambots do attack the contact forms on websites, in the hope that at least one person will get (and read) the spam.
So it's possible that the bot simply makes no difference between a contact form and a newsletter subscription form here, because it was not taught to discriminate, and no "preselection" of victim sites was done by the spammers. This could be the result of an "autodiscovery" process: the bot crawls the web, and indexes everything that looks remotely "spammable".
Since your form contains at least one field whose name or placeholder suggests that an E-mail address is expected as input, this could be enough for a bot to flag your site as a valid target and attack it - blindly.
Finally, there have been incidents in the past where contact forms were hijacked to spam a lot of arbitrary recipients through null byte injection. The same technique can also be used against file upload forms to bypass restrictions on file name or extension for example.
I believe this type of attack has become less common nowadays because of increased awareness, and some libraries like phpmailer have built-in protection against it, but it still possible to be vulnerable if using old code and not being aware of all the possible pitfalls and best practices.