This is why a 17 year old's opinion doesn't matter.
Countries require borders because things like infrastructure, structural planning, educational planning and town planning require reliable data of who will live where in the next 20,30,50 years.
Without the ability to predict that you will inevitably run into the mess we see now with water, electricity, roads, ports, railways and housing.
We already have an incompetent government, lump in millions of illegal migrants and you end with a humanitarian crisis, starvation, pandemic and societal collapse.
I support immigration, both legal and illegal. I love immigrants. Do not lump me in with the xenophobic South Africans