Because every time an account logs onto a computer, it leaves traces. Some ephemeral in memory, some permanent on disk. It can be Kerberos tickets, process tokens, domain cached credentials, hashes or even clear text passwords in memory. It's common practice in a lot of organizations for administrators to log on to random workstations to perform whatever task they need to do.

Or there is a service running in the context of a service user domain account. Or the password of the local administrator account is identical on all systems, which was very common before LAPS became a thing.

Yes, if you do everything perfectly and always go by best practices, none of this should be relevant, but most people aren't doing everything perfectly all of the time.

To access any of these things, you need local admin permissions. Then you can reuse them to log on to other systems.