Countries tend to only allow immigrants that are essentially guaranteed to benefit the country. For example, in Canada and many EU countries, immigration generally starts with a work visa, but you can’t just apply for any old job, it generally needs to be a job that the country is having trouble filling. This is enforced on the side of employers by requiring that they must search for applicants within the same country before opening the position up internationally.

In the much rarer instances where countries agree to give you permanent residence without a job first, it’s almost always exclusively for highly skilled or educated individuals. For example, most countries tend to let you stay if you finish a Masters degree or PHD in their universities, or if you have an impressive work history in your home country in a white collar field like STEM.

Even rarer than the first two but still exists: “pay the government an obscene amount of money and we’ll give you a residence card.” For example: Austria and Malta. Canada also used to do this but has since stopped offering it as an option.

Basically, immigration policies are generally “unless you can be of significant economic benefit to us, fuck off.” This is actually very detrimental to the countries where immigrants originate, because of the brain drain caused by wealthier countries skimming away the most skilled people, when those countries are already less developed and having trouble developing their own skilled industries. This is also why many countries actually have restrictions on who can immigrate away.