There could be a few reasons why you might not be able to sign up directly on a website, and would instead have to use a Google account. One possibility is that the website requires all users to sign up with a Google account in order to verify their identity. This helps to reduce the amount of spam and fake accounts on the site.
Another reason could be that the website is integrated with Google's sign-in system, and thus only allows users to sign in with their Google account. This makes it easier for users to sign in, as they don't have to remember another username and password for the site.
If you're still having trouble signing up directly on the website, you can reach out to their customer support team for assistance.