Having unconfirmed Subscribers isn't always a bad thing. There are several reasons why this can happen.
When you change your subscriber dropdown menu to "Unconfirmed Subscribers", there's a list of subscribers who never confirmed their subscription. This can mean a few different things and normally is nothing to be alarmed about.
The signups might have been added by a bot
You could be experiencing something called listbombing. This means subscribers might be added to your list, but they didn't actually sign up to be there. In this case, they won't confirm their opt-in, and will be added to you list of unconfirmed subscribers. Listbombing can start to result in elevated complaints, which hurt your deliverability.
The best thing you can do is to ensure you are using double opt-in and reCAPTCHA. It's a good thing that these subscribers are unconfirmed, so they don't join your list and receive any more unwanted mail.
The subscriber forgot to confirm their opt-in or they changed their mind
It's easy to forget that subscribers are real people like you and I. They may sometimes opt into your form and forget to check their email right away to confirm their subscription.
When they think about it again, the confirmation email may be lost in a sea of other emails they've received. They also could've opted in and changed their mind. Some people act in the moment and decide after the fact they'd rather not add to their overflowing inbox.
NOTE: If your new subscriber has not confirmed their subscription, there's nothing more you can do to reach out to them. This is the beauty of the double opt-in because it keeps your list full of engaged, active subscribers who are eager to receive your content!
Strict filtering settings might have caused the confirmation email to go to their spam folder
Some subscribers could have stricter spam filters depending on the email provider they use. For example, government employees and universities often send any external email to the spam folder.
It's a good idea to add a blurb at the confirmation step encouraging new subscribers to check their other folders for the confirmation email.
You can also ask subscribers to allowlist your sending address.