I am confused. the situation is exactly as you describe, modulo the word "default": zinc oxide sunscreens are available, chemical sunscreens are also available. And I'm not sure what "default" means in this context.

Anyway, if you spend a lot of time in the sun or at the beach the functional problem with zinc oxide is pretty clear: it's harder to spread and put on. This might not seem like an issue but if you spend a lot of time dealing with it, the annoyance adds up.