Labor costs- Without borders, there would be no place to hold people to cheap labor. In that situation, the full cost of labor would have to be paid globally. Even if you kept most borders but eliminated borders within the global south, it would be problematic. In the current system, companies can force workers in thailand to eat the costs under the threat of moving elsewhere. If Thailand doesn't like it, they can simply move to Vietnam. Without borders between these countries, there would be nowhere to run and the companies would have to pay the true cost of labor.
Environmental costs Where would we send our trash and pollution if there were no borders? It is not a coincidence that the countries who benefit from pollution are not the countries who suffer the consequences of it. We are consuming the environment without having to assume the costs of that consumption.
Hmm, lets take a deeper look at your arguments.
Labour costs and environmental costs, they very often are the same. Garbage is sent to the locations where labour is cheap, in this way it can be sorted cheaply. Good example is Bangladesh and ship breakage.
I highly doubt that the main reason is economical one. Because cheap labor is geographically restricted. Again good example is India, there are not much options for people living there get to Europe or USA, because they just dont have the means to travel that far.
Argument for economical benefits without borders- most countries are trying to get trade deals with each other to overcome economical hurdles made of border restrictions.