is it really necessary to keep using new version of postgres and to keep developing them? is it not done at this point?

Yes, not only is there always improvements to existing functionality possible due to changes in improvements in general software engineering and mathematical methods beyond it or supporting new hardware features.

Generally, software engineering like other industries is evolving and there is need for different facilities. Consider graph and vector databases featuring, as well as json columns as examples of things you that are relatively recently in common use.

Yes. There's always room for improvement.