Which economists? Economists generally support freer trade, more open borders and the like because they see trade as fundamentally not a zero sum game: if two parties are willing to make an exchange, it must be because the exchange somehow makes them better off than they would be without it. All of developmental economics is built on this: it's why the majority of the world's population enjoy an incredibly greater standard of living than they did 100 years ago. Everyone is better off.
Which is not to say there aren't situations where economists recognise trade-offs. For example, https://en.wikipedia.org/wiki/Factor_price_equalization: a commonly accepted economic theory that proposes that international trade between poorer and richer countries will make unskilled wages in the poorer countries higher and in the richer countries lower.
