to play the devil's advocate, if more people wanted to visit the US than the other way around, then it's not "disadvantageous" for the US to do this.
It's disadvantageous for the US if their citizens have to go through more bullshit whenever they are visiting another country. Regardless of how much they subject people going to the US, or how many people travel either way.
It's a lose lose pissing contest. The reason reciprocity is exercised is to discourage this kind of thing in the first place.
That would only be true if the per capita advantage to the US of doing it is at least as large as the per capita disadvantage of having it done to US citizens. Which it isn't. The value of doing it is negligible and the cost of having it done to you is significant.
Take student visas. Sure, you could have a student come to the US, finish their education, and go back to their home country, "stealing" knowledge from the US to benefit their own country. Or they could find a job in the US and/or start the next trillion dollar company since the opportunities in the US are better. Satya Nadella traveled to the US for a university degree and ended up at Microsoft, where he led business units bringing in tens of billions, and under his CEO-ness he increased the value of the MS stock from around $40 when he became CEO in 2014 to $477 today, making it one of the first trillion dollar companies in the US.
But that wouldn't have happened if he didn't get a visa. Neither would Tesla (Elon Musk, migrated from South Africa on a student visa), netiher would Google (Sergey Brin migrated in from Russia, Sundar Pichai migrated on a student visa from India), etc.
I just don't understand it.
This is, of course, immaterial to your main point: we can point to many actual contributions from migrants, such as maintaining infrastructure, providing food and education, and technological advancements.
Yes, it does, it’s called the wealth effect. This is beyond firm effects that stem from lower costs of capital.