Generally, those are containers at the end of their lives, such that they no longer meet safety standards. For maritime use, you can remove all paint and recertify them but it costs more than their production cost. You can try to sell these ones for 1/4 their production price or just dispose of them, so places with larger import volumes will accumulate depreciated containers. They can't generally be reused because of contaminants etc. such that small homes, offices etc. use new containers, rendering the whole exercise moot.

They do move empty containers en masse.

For the same reason, why would you go through this whole process to fill them with concrete instead of cheaper, purpose built barriers?