Here's why:

* Naturally Occurring Actinides: Only thorium (Th) and uranium (U) are found in significant quantities in nature.

* Synthetic Actinides: The rest of the actinides, from protactinium (Pa) to lawrencium (Lr), are all synthetically produced in nuclear reactors or particle accelerators. They are created by bombarding other elements with neutrons or other particles, leading to nuclear reactions that result in the formation of new elements.

Key Point: While traces of some actinides heavier than uranium can be found in uranium ores due to radioactive decay, these are in extremely small quantities and are considered synthetic for practical purposes.