That's not really an excuse - you really only need 3 page loads:
* initial page provided by the service you're logging into - this gets your email address so it can lookup your account and determine which SSO provider to redirect to
* actual login page served by your SSO provider - here you authenticate to the SSO provider. It can occasionally cause another page load to get your 2FA code if configured, or go through further identity checks
* final "page" that consumes the query parameters sent back by the SSO provider - this is often just a 302 redirect to the home page but sets a session cookie.
The main problem is that all these pages are super bloated, with tons of unnecessary JS and BS. All the code for login page that takes a username and password should be able to fit entirely on an A4 sheet of paper - it's literally just an HTML form and a few lines of CSS.
Furthermore, even beyond inter-company SSO, there are shitty companies out there which use such flows internally even though everything is part of the same security domain, hosted on the same infrastructure and thus can be hosted on the same top-level domain and use a single session cookie. Microsoft is a pretty bad one - Teams for example will use a redirect to some other Microsoft-owned domain to get your (already existing) Office 365 session; this is completely unnecessary, they can host all those things on the same top-level domain and reuse a single session cookie seamlessly.