Open borders is like removing the doors from your home
When we put locks on our doors and fences around our home, we do so with the intention of protecting what we have created. A country has a culture that supports what it created, and borders help to protect that.
Wealth and culture are not the only things worth protecting by controlling borders. Any human investment that has resulted in some tangible creation that is then desired by other humans (or non-humans) will need to be protected. This results from base drives that affect behaviors. Some of these drives are directly related to physical survival, hunger, thirst, cold, etc. and others are emotional, greed, lust, laziness, etc. For a stable society to exist, the physical needs must be met, and the anti-social emotional drives need to be managed. Culture is one factor that contributes to this control. Wealth and culture are both examples of tangible creations that require an investment to create, and therefore induce a desire to protect.
Analogizing ones home helps to make the argument for borders more tangible. We all understand that our homes are places that are created, and require a continued investment to maintain, both emotional and physical. We all know either from direct or indirect experience that not all people will respect the investment you have made, and given the opportunity, will take it, thereby enjoying the benefit of your investment without the effort of creating it themselves. So we make a law, and invest in a police force to enforce the law, and execute consequences for breaking the law, all in order to discourage people from following our innate human drive to maximize gain while minimizing effort, which is at least partially driven by greed, lust, laziness, etc. The severity of the consequences are driven, at least in part, by what the culture, rather than individuals, values most. The fences and the doors are reminders that there is a law, and there are consequences for breaking the law. All this because our homes have something of value that we want to protect, in the same way a country has something it wants to protect.
Borders around a country are analogous to doors on your home.
Your home is something that you have created and has value to you.
You don't let just anybody that shows up at your door into your home.