Unable to access iOS 18.7.3 for security updates

This is a user to user only forum. We can't answer why questions. But we can look at history and that tells us that every year, after a period of time, Apple stops signing the previous year iOS for any iPhone which is capable of running the current year iOS. Your tag line suggests you own an iPhone 13, which is fully capable of running the current iOS 26.2. If you want the latest security updates and bug fixes, this means you'll need to update to the current iOS version.

We've been on iOS 26 since it was rolled out in September. It has NOT been buggy. You'll ALWAYS see posts from people who are upset that things change with the new iOS version. That doesn't mean the vast majority of people are having issues. People only tend to complain on forums. People usually don't join a forum and say, "hey, everything's fine."

Sure, there are changes. Liquid Glass is one of the big changes. But it too can be adjusted, which is why I wrote this --> Liquid Glass - Love It Or Hate It - It's … - Apple Community

And you can arm yourself with some knowledge of the changes in iOS 26 by reading this --> OS - iOS 26 - Apple

Why questions on Apples decisions cannot be answered by us as we are simply users like you and you are not speaking to Apple here on this user to user support site for any answer. You can send Feedback to Apple here:

Product Feedback - Apple

These are the devices Apple has issued the iOS 18.7.3 update for, so that is why it is not available on your iPhone 13.

• iPhone XS, iPhone XS Max, iPhone XR, iPad Pro 13-inch, iPad Pro 12.9-inch 3rd generation and later, iPad Pro 11-inch 1st generation and later, iPad Air 3rd generation and later, iPad 7th generation and later, and iPad mini 5th generation and later

Apple security releases - Apple Support

It has always been recommended by Apple to keep your devices updated to the latest release versions of the OS for security protections. On your device that is iOS 26.2. I currently have it installed on 3 devices in my family without any trouble and also consider the many millions more that have it installed. You are just prolonging the inevitable and if your device needs to be restored for any reason, including something as simple as a forgotten passcode, your only option will be iOS 26.

The only iPhones that have access to iOS 18.7.3 are the iPhone XR, XS, and XS Max – the ones for which iOS 18 is the end of the line. Some of Apple's initial support documents implied that iOS 18.7.3 was available for all iPhones that had been capable of running iOS 18, but it turned out that it was the documentation that was in error (much to the dismay of those who were hoping that it was correct). Apple then revised the documentation.

Servant of Cats wrote:

The only iPhones that have access to iOS 18.7.3 are the iPhone XR, XS, and XS Max – the ones for which iOS 18 is the end of the line. Some of Apple's initial support documents implied that iOS 18.7.3 was available for all iPhones that had been capable of running iOS 18, but it turned out that it was the documentation that was in error (much to the dismay of those who were hoping that it was correct). Apple then revised the documentation.

For a couple of months after iOS 26 was released you could still keep iOS 18 and continue updating it; in Settings/General/Software Update you had a choice of updating to 18.7.3, but that time is long past.