Many people from around the world must agree, because so many immigrate here.
No one is saying that the U.S. doesn't have a lot to offer. It does. There certainly are some great advantages to living in the States. But talk to a lot of those people who move to the States and ask them if they think the U.S. is the greatest country in the world. If they are able to get past being guarded about answering a loaded question, they will very often tell you that they miss a lot of things from back "home" that you just can't find in the States. Most reasons to come to the States are purely financial. Usually not much more than that. Oh yes, there are those who are escaping persecution and displacement, but those aren't the majority, and even they will tell you how much they miss their home coutnries. Most people just want to have more money.
As to great numbers of immigrants? More people try to get into the EU than into the United States. Far more. Australia does a fair job, proportionately, of taking lots of people in, too. So does Canada. And Brazil. In fact, most of Latin America is made up of immigrants. My wife is a second generation Japanese immigrant to Brazil, one of over 2 million in Sao Paulo alone. She loves Brazil in spite of all its problems and would NEVER consider moving to the States. What did she do? She came to Japan, of all places (Japan's immigration policy hardly allows anyone at all in, and their attitude toward non-Japanese is abysmal) to make money, not because she loves Japan (she dislikes it intensely). My Filipino grandfather immigrated to the States in the 1910's because of the grinding poverty back home (caused in great part by the American, and earlier the Spanish, occupation of the Philippines) and lived in America all his life, became an American citizen, served in the American navy, but all his life he venerated the Philippines and tried to find a way to get back home without causing hardship for his family.
The stories about success and what people get for their hard work are not as simple and straightforward or rooted in patriotism as a lot of people would have us believe. Life is never that simple. Most people, in spite of working like dogs all their lives (like all my grandparents did... my German grandparents... factory workers who lived through the worst of the Wars and the Great Depression... ended up with much better care and benefits than my American grandparents ever came close to) never get close to getting out of the rat holes. And anyone here who suggests that a non-white person, especially up until twenty years ago, had the same opportunities as a white person, needs to seriously take some time learning about what the lives of non-white people are like, especially in places like America. A white person simply has no concept of the reality of being non-white. Period. You don't get stopped by the police for walking in a white neighborhood. You don't subtly get asked to take the enlistment test again (like my father was) because you did too well on it the first time. You don't have people at parties ask you to fetch them another drink. You don't walk into a job interview and never get asked a single relevant question because your name is Spanish and you look Latino. You don't have employers lean across to you and, in all seriousness, ask you how much your sister costs and how great Filipinas are for sex tours. You don't have women leave a bar in disgust, calling you a F***ing Iranian. You don't get subjected to constant airport security searches, having everything you carry strewn about the table, while not a single other passenger, all white, even gets asked to stop. You don't have white doctors, working in a research facility where you are working part-time as a glass washer, refuse to sit next to you in the lunch room, because, to them, you are just a lowly immigrant (I had a Master's in Architecture), or white Harvard Medical School graduates at a party take one glance at you and turn to another person to talk. My father, an extremely intelligent man who could probably out debate anyone here in these forums, had to leave the States in the 60's in order to be able to go beyond the restraints of being Filipino/African-American and achieve the success in the publishing industry that he dreamed of. He found it here in Japan, reaching the United Nations, where he became head of one branch. He never left Japan because he feels he would never have been given the time of day in the States. Just imagine that... a country as prejudiced as Japan gave him opportunities that he would never have been able to get in the States.
Sorry... I have to strongly disagree that everyone, anyone at all, has the same opportunities in the States. That is simply bull pocky.