I don’t understand why they can’t just replicate the option they’ve already done work for.

I would bet $5 it has to do with account management.

Consoles have multi-account built-in as a first class citizen. The game console operating systems are built from the ground up to support multiple players using different accounts.

Battle.net on PC has been built around a single account for 25 years. Should it be upgraded so that multiple users can log in on a single PC so they can play games and maintain their progress? Maybe! But I guarantee there is code sprinkled all over the codebase that assume a single account is active at any point in time.

Is that a good enough excuse? Meh I dunno. But the answer to "why can't they just replicate" is "because it's take more work than you'd think" and my $5 says the reason has to do with account management. Could be wrong! It's a moderately educated guess.