From a US perspective, I think it’s because people enter on travel visas and do not leave.
I had a coworker from India who was a naturalized citizen for 5-10 years and he was trying to get travel visas for his parents to visit. He had applied many times and been denied each time. We talked about it and he said that the reason given was that the state department considered his parents a risk to not leave at the end of their visit. Comically, when I complained about how that was wrong and his parents would obviously go back he said “100% they will stay, I don’t really blame the visa people. My in laws stayed on a travel visa years ago and are still here.”
There’s also a reciprocity issue that’s sort of chicken and egg. India didn’t grant visas easily back in 2005 because the US didn’t grant their citizens visas.
The question isn't that much "why don't we accept all travel visas" but "why don't we accept all immigrants".
People are talking about the "risk that they stay" like it's something bad to have too many immigrants. And it ended up like some common sense idea. But what if it's not?
Did California or New York suffer from getting people moving from all the "country side" state? Did the US suffer from their immigration waves when they had open borders, and all you needed to get in was a medical exam on Ellis Island?
I think there’s pretty common and clear data on why unlimited immigration is harmful to the receiving country [0] (tl;dr; lowers wages and increases costs for non-immigrants).
The question is whether it’s worth it for the obvious benefit to immigrants. So it may be a net benefit when factoring the improvements to people who immigrate in.
The US benefited greatly from open immigration 100+ years ago because they weren’t that great and there wasn’t much to lose. So it’s apples to oranges of open immigration then to open immigration now. Back then there were no government services and land was literally given out for free.
Same thing happened to my country. In the 90's, the US was a lot more welcoming. But because a lot of tourists stayed, it is a lot harder now to get a tourist visa.
I had a coworker from India who was a naturalized citizen for 5-10 years and he was trying to get travel visas for his parents to visit. He had applied many times and been denied each time. We talked about it and he said that the reason given was that the state department considered his parents a risk to not leave at the end of their visit. Comically, when I complained about how that was wrong and his parents would obviously go back he said “100% they will stay, I don’t really blame the visa people. My in laws stayed on a travel visa years ago and are still here.”
There’s also a reciprocity issue that’s sort of chicken and egg. India didn’t grant visas easily back in 2005 because the US didn’t grant their citizens visas.