alt Hacker NewsIt's not even that it's hard to build a modern account system. It's that if you put your site behind a private / custom account system, and you post a link to it on HN (for example), everyone and their mother complains that it's requiring an email address and password. But if you put the same site behind a Google auth, most people wouldn't think twice to click the button.
I worked on the Google account system for a few years. I'd say it's a lot of work to beat it. Not necessarily "hard" for a team with the right skills, but certainly a lot of sweat, blood and tears.
A modern account system is expected to have, in rough implementation order: email confirmations, password strength checks, password reset emails, forgot password flows (=advanced ID verification as otherwise this becomes a backdoor into accounts), user profiles (+avatar image upload/recompression/hosting), usernames independent of email addresses along with ability to change usernames later, password brute forcing blockers, bulk signup prevention (=solid bot detection), abuse controls (can easily become a team of people), 2FA (SMS), 2FA (authenticator apps), 2FA (backup codes), 2FA (voice calls), 2FA (passkeys), 2FA: recovery when both factors are lost, enterprise SSO integration (SAML), enterprise SSO (Active Directory), fast global signout support (much harder than it looks), cookie theft mitigations, heuristic online login risk analysis to catch cases where an attacker knows the right password via phishing, support for signing the user in to multiple domains, audit logging so users can review their own sign-in history, age verification and restriction support, and possibly support for being logged in to multiple accounts in a single browser session.
Oh, that all has to be HA, and the account system is the keys to the kingdom so the security requirements are the strictest of any part of your system.
You might say we don't need all of that, but expectations rise over time. Maybe 20 years ago you could get away with a simple account system and an automatic forgot password flow that just assumes the user still has access to their email. Maybe today you still can write a simple system, if you don't expect to have many users and are willing to implicitly delegate identity to webmail providers anyway (the moment you assume the user has access to a secure email account you're basically doing Sign In With Google anyway for 90% of users). But if you roll your own accounts, and then the user gets phished and someone logs in from an obviously suspicious place with the right password, they won't say "yes that's my fault" anymore, they'll say "Google could block that log in, why didn't you?" or maybe "Why didn't you support 2FA? It's your fault".