This is because of Enterprise Single Sign On. You can try this for yourself by going to https://gmail.com/ and enter the email of a public person at a large org, for example the CEO of Doordash (`tony@doordash.com`). After you enter the email, you get sent to Doordash's employee portal to authenticate. Based on the email you provide, Gmail has to figure out if you need to provide a password to gmail itself or if the email authenticates another way.