Nomenclature is important. As the joke says, naming things is one of the only two difficult problems in computing. [1]
A good password is a long string of random characters, and an API key is a long string of random characters, but they are not the same because the semantics are different. Passwords are typically chosen by customers, changed (let's be honest) very rarely and on a haphazard schedule, often reused across a bunch of different systems no matter how frantically we wave our hands, and (most important of all) have "user" semantics and scope: a password lets a user log in and do logged-in-user stuff.
Whereas an API key can be issued by the system, and is thereby guaranteed (assuming a clueful programmer) to be long and random and unique and not shared with the customer's Gmail account and bank account. You can expire it often. You can issue more than one per customer. (With passwords, this is theoretically possible but practically impossible; customers barely understand how to handle one password.) You can issue it to entities that aren't themselves customers, and that don't have "user" semantics. You can scope it to particular uses, issuing a key that is only good for checking balances but not for initiating transactions, for example.
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[1] The other being cache coherency. And avoiding off-by-one errors when enumerating lists.