@tindall@cybre.space a generous answer would be that the access point for existing users is typically something other than the front page. for example, if you try to access your dashboard, and you aren't signed in, it will prompt you to sign in, whereas if you navigate to the front page it will ask you to sign up.

another answer is that people have a negative emotional reaction to being asked for things they don't have (like an existing login). if your sign in is more prominent than your sign up, new users will be discouraged and feel like your website isn't interested in them. so there is an interest in only asking for an existing login from people who you know HAVE an existing login, hence a separate page.

however, the most realistic answer is that some popular people started doing this, and most designers are programmed to always favour convention, so everybody followed suit without really thinking through the ramifications. 🤷🏻‍♀️