Money.

Also it kinda depend on how much you trust the website security and how much precaution you have. For general public who don’t really know how to protect themselves against hacking and databreach(those who might not know the existence of password manager), the option of letting a giant corpos handle the login is much better than to just blindly trust the website.

Also money.

Also the website might not want to build and maintain their own database for this(which cost money), so they outsource the login to other company.

And also money.