Can anyone enter the U.S legally?
They're all wrong. There is no way anyone would do all the prep work, applying for an H1 Visa, paying the thousands of dollars to fast tack it so that it might be completed in nine months or a year. Just come over, fly into any airport without any passport or documents of identification and speak american english, no accents. Tell Customs that you got really drunk and flushed the passport by accident. You're from East LA.
There are tens of thousands of people entering the US properly every day ( Legal Disclaimer : I am not going to say legally because I have never met any of them to vouch for their integrity and honesty.)
There are close to the same number of people opting to not follow the established process which is neither good or bad for them but does create a drain on social services and other resources. The funding for the Food Stamps and other assistance programs do not have a finite cache of funding.
The better question to ask might be: Why did the native americans insist the pilgrims to have a return ticket before allowing them to enter?
Sure. You want to visit? Get a visa and fly over for a bit. Want to study? Apply for a program and a student visa. Want to immigrate? Apply for residence and/or work. The first one is reasonably easy to obtain. The other two require more work and have more limited access, as one should expect. Immigration, if you are uneducated or have no family, is limited by a complicated system of requirements and quotas. It's not impossible, just not the easiest thing. This is the case for entering other nations not in political pacts.
Europe has the European Union, the US has the very small Compact of Free Association between the US itself, the Republic of the Marshall Islands, and the Federated States of Micronesia. The members of these nations can easily emigrate to each other.