You have to remember that this list isn't sorted by the total number of people who'd want to move to a specific country (in which case USA would be #1). It's sorted by its population percentage difference, so what it's saying is if we had universally open borders the population of Singapore would more than double. That wouldn't happen in real life, because its already high real estate prices would skyrocket, creating natural economic barriers to entry.
It's also a matter of perception and other biases. Muslims want to move to the best Muslim-majority country, not necessarily the best country overall. A lot of Jews in America say "I really ought to move to Israel" just as a cultural thing (perhaps to alleviate some psychological guilt that was brainwashed into them) even though they have no intention of moving. A lot of people hold up Sweden as the example of an "idyllic" "welfare" state, which overshadows less famous countries that are roughly on the same level. Etc.
Probably the biggest mystery on that list is the under-performance of Chile. It only started to top the lists of economic freedom a few years ago, so the full effect didn't sink in yet. Being the best Latin American country still means a lot of people want to move to the USA. It also has its share of socialists who think Venezuela would give them free lunch. Things like perception of climate and earthquakes also play a role.