Basically though, people move for lots of reasons, and the reasons and rates of movement can vary depending on national or global events. It helps if you can get them to narrow down a specific period of migration. That said, a lot of the time liberals are thinking about people leaving the USSR in the late 80's and early 90s as it was getting turbo-fucked by neoliberalism and being actively destroyed -- which, shockingly, was the major driver of people leaving at that time.
If you think a cheeky response would be suitable, "If America is so great, why are they moving to mexico in record numbers?" :
https://www.businessinsider.com/number-of-people-moving-from-us-to-mexico-2019-5
https://mexiconewsdaily.com/news/us-citizens-moving-to-mx-record-numbers/
Basically though, people move for lots of reasons, and the reasons and rates of movement can vary depending on national or global events. It helps if you can get them to narrow down a specific period of migration. That said, a lot of the time liberals are thinking about people leaving the USSR in the late 80's and early 90s as it was getting turbo-fucked by neoliberalism and being actively destroyed -- which, shockingly, was the major driver of people leaving at that time.