Your body does have muscles that run through it that evolved to sit under tension all day. But one of the reasons why you end up with posture problems because of sitting is because the body recruites muscles that were normally used for other purposes to do this instead. Hip flexors, hamstrings, etc...you sit in a chair and your body uses this muscles to keep you upright, over time these muscles start to become physically shorter to the length they're sitting in when you're in the chair.

Ergonomic chairs are designed to support your body in such a way that you don't need to use these muscles. Having ideal posture and muscle activation is the other way people avoid these issues, if your body is capable of sitting in a chair and not needing to engage them in the first place.

An exercise ball though provides no stability and greatly increases your need to keep these muscles engaged to stay upright...not the way you want to go for better posture and less issues from sitting.

An exercise ball can have different improvements, mostly in not smashing your glutes and hamstrings into a hard chair surface, but so can a better chair.

At least this is my understanding of the pros and cons of different chairs based on reading combined with personal experience.