The biggest problem with having separate accounts for everything is that a lot of the users will make their own "wish it were SSO" by setting their passwords in the systems to the same value. Then, when the weakest system is exploited, the attacker gets credentials that are valid across the organization. Yes, they should be using a password manager with unique, random passwords for each system, but realistically a good chunk of larger organizations' staff are not going to do that.

Some other headaches:

Having decentralised authentication means that onboarding and offboarding need to have a bunch of tedious manual steps, or custom automation.

Whoever does user support for the organization has to be trained to reset passwords/unlock accounts in a hodgepodge of systems.

Any security controls the organization wants to implement need to be reimplemented or approximated in a bunch of different systems. E.g. if there are regulatory requirements for account lockouts, time between explicit reauthentication, etc.

It becomes much more critical to collect the authentication logs/event data for all of those systems, and harmonize its formatting with everything else so that the security ops team isn't maintaining separate monitoring/alerting rules for every system.

For large-scale systems, there are also at least theoretically performance advantages to the kind of signed ticket approach that SSO mechanisms tend to use, versus having to do database lookups of session IDs or verify a password. It's possible to do that without SSO, but if you're going to the trouble of implementing that kind of mechanism, you're most of the way to having SSO anyway, and might as well just finish the job IMO.