Also, the ones abusing the system tend to know it better: often it's their jobs to figure out how to work it to get what they want. The people who just want to use legitimately often it don't have the time and experience to learn it.
(You see a similar thing with benefits and healthcare: often attempts to crackdown on people abusing the system just make it harder for legitimate users)
If you are from US I want to let you know about a scam. Here in asian countries people with good enlgish accents are recruited to pretend to be US citizens and claim benifits that are unused by the real people. There is a whole proper process of how this works. The recruit is given full identity information and a software to make the call appear from the US. The officials on the US side are in on this and get a share for each successfull claim.
(You see a similar thing with benefits and healthcare: often attempts to crackdown on people abusing the system just make it harder for legitimate users)