Sims in The Sims 4 have some autonomy, but they’re not nearly as self-sufficient as real people. Even with Full Autonomy enabled, they still need a lot of direction from the player.
For example, if the toilet breaks, your Sim may simply let their Bladder need drop until they pee themselves instead of repairing the toilet or calling someone to fix it.
The same can happen with broken showers, bathtubs, and other household objects.
Their autonomy is mainly designed to keep them functioning, not to make sensible long-term decisions.
With Full Autonomy enabled, Sims will generally take care of basic needs and perform some everyday activities.
They can go to work, eat, sleep, use the bathroom, cook, socialize, and interact with objects around the house.
However, they won’t usually take major life decisions on their own.
They won’t decide to get married, have children, change careers, pursue a particular hobby, or work toward a specific aspiration goal without your input.
They also won’t reliably handle things such as paying household bills.
The Sims 4 is designed around player control. Autonomy gives Sims the ability to make smaller decisions, but the game still expects you to direct the important parts of their lives.
That’s why you can leave a household running for a while and come back to find that everyone is technically alive, but their needs aren’t necessarily in great shape, and very little meaningful progress has happened.
If you want Sims to behave more independently and make more decisions without your input, mods such as MC Command Center (MCCC) can give you much more control over their autonomy and household behavior.
Without mods, though, some hand-holding is simply part of playing The Sims 4.