Well, it’s often a matter of the materials as well as the environment. Coins with a high silver or gold content survive well, and bronze coins survive well if the environment is favourable. Also, most of the coins in the picture were also valuable in ancient times; it makes sense that people would try to keep them safe, maybe hide them well in places where they weren’t often exposed to modern chemicals. Or they’re dropped in the desert where conditions are very favourable.

Ancient coins were struck, btw, or very rarely cast.

Sounds like you’re about ready to start collecting ;)