> Why is it that everyone seems to be okay with someone having no human rights until they are 18? In the USA people under 18 are viewed as property.

For starters, because these statements are not true [1].

[1] https://en.wikipedia.org/wiki/Timeline_of_young_people%27s_r...

While you are correct it is not everyone, as an outsider it is still striking how different kids rights are viewed in the states compared to Europe/Oceania. The OP is correct that you hear the "it's my child I have ultimate control over it" is much more expressed by US Americans than pretty much anywhere else. So much so that when there is discussions of restrictions this is considered to impede on the fundamental freedoms of the parents (yes the irony).

>...you hear the "it's my child I have ultimate control over it" is much more expressed by US Americans than pretty much anywhere else...

Probably true, but technically because the sentiment is expressed in English. Non-western cultures often take some things a few steps further.

Yet many European countries have no concept of emancipation.

Where does "not ratifying the UN convention on rights of the child" fit in that timeline?