Because those people have zero idea about the world outside and dont realize that with all its faults, US immigration system is still one of the most permissible and open out there.

Try immigrating to Germany and becoming a citizen, for example, and you will quickly find out that unless you have a citizen relative or you are married to a german citizen or a few other very specific cases, you are out of luck. Like, completely:

And that lack of knowledge of the world outside is pretty much exactly what drives them to be so hateful about their own country. Went through the same motions myself, but the other way around. Thought US was a rosy paradise back when i was a teen, actually immigrated, and then realized that it wasn’t all so rosy at all. However, looking at it objectively, i will take this over what i had before without even a second thought. And now i am seeing those people have the same “rosy” view, but regarding the abstract all-encompassing world that isnt the US.

Note: visiting a country on a trip gives you zero idea what it’s like to actually live there, and pretending otherwise is silly. What you see as a tourist has almost zero relationship with how it would actually be like living there full time as an immigrant.