I'm pretty sure Google does it to set all the appropriate cookies on the right domains. Microsoft probably has similar redirect flows.
You can either do the redirects all at once on login or do them once you use the service first. Since login is already a time-consuming process (username, next, password, next, 2FA, next) I think you may as well take a second to add the redirects and be done with it.
It doesn't make much sense for Google work accounts but it makes sense if those are a minority on the platform. They could definitely patch this out, but then again the login process is something that takes a second extra every month or so, so who really cares.
What does bother me is how every service wants you to enter your username and password separately now. Autofill gets confused and sometimes even stops working because the stupid hidden input fields for the password don't get shown until you click the magical "next" button, just in case you need a special third party auth service.
Either decide that work accounts are important and take out the extra YouTube redirect, or decide they aren't important and let me fill in my username and password on a single form. Both make complete sense individually but combined they're just a massive waste of time.