pokemon with around 580-600 base stats are fully evolved Pseudo-legends.

You cannot however obtain the prior evolutions.
As they all have evolved and cannot breed/sterile with any pokemon, while pseudo legendaries still have babies.

the reason they may not be able to breed with ditto is that perhaps they were fully evolved at a point when ditto was synthesized. And while mew contains their dna ditto may not.
Or perhaps they reproduce assexually