2022 me agrees with you, but 2003 me getting an invite to GMail when it was a brand new service and essentially a completely different company with a different landscape didn't know better. Now I have nearly two decades of accounts and things tied to GMail =(
I think the main problem with many people isn't so much the archive of email that they would lose from not using GMail anymore, it's the many years of accounts that are authenticated with it. There are literally hundreds of services I have that are registered to that e-mail address now.