Why doesn't someone just arrest these criminals?
That's a fair question and I'll try not to write a long lawyer answer. The short answer is that there isn't really any way to do that.
Let me give an example. Let's say there is a bank near you and someone robs the bank. There is a "locus delicti", which means the "place of the wrong", according to Black's Law Dictionary. When the bank is robbed, there are police who can respond to the place of the crime and there are things they begin to do to find the person who robbed the bank. If arrested, that person is tried under the applicable law in that jurisdiction (state, province, or country).
But in cyberspace, where is the place of the wrong? Where does the crime occur? As an example, let's say a man is scamming from an internet cafe in Lagos. He scams people using an email provided by yahoo.co.uk. He scams and steals money from someone in the U.S. Where does the crime occur? In Nigeria where the scammer is physically located? In the U.K. where the email provider is hosted? Or in the U.S., where the victim is located? This is a crime of international proportions, and there isn't any clear law that is applicable on an international level. There are no international anti-scam treaties, although it would be nice if there were.
The scammer has broken the law in Nigeria under section 419 of the Nigerian Criminal Code (see above^^^). He has broken the law in Nigeria but someone has to arrest him, put him on trial, and convict him for that crime. Unfortunately, the problem is so large the Nigerian police are not able to cope with it, although they do occasionally make arrests for scamming.
It would be harder to argue that a crime has been committed in the U.K., although laws there might cover internet crime such as scamming. But the man in Lagos would still have to be arrested and then the U.K. would have to request his extradition to the U.K. to face criminal charges there.
The same would apply to the U.S. where the victim is, in this example. Under U.S. law, the Americans often argue what is called the "effects principle", that is, although a crime starts outside the U.S., the effect of the crime is inside the U.S. An agressive U.S. prosecutor could charge the Nigerian with a crime, but again the scammer would have to be arrested and extradited to the U.S. to face trial. This can be done, but it requires a strong criminal case and a lot of cooperation between the Nigerian and U.S. governments. That might or might not happen.
So there is the problem of where the crime happens. There is also the problem of getting law enforcement agencies to take scamming seriously. In many cases, they simply don't, or they can't do very much. If someone sends $100 to a Russian romance scammer, what police force has the time to investigate the crime? I don't know what the threshhold level is for getting law enforcement to take scamming more seriously. $1,000? $5,000? $10,000? $50,000? Still, some law enforcement agencies in some countries are beginning to take this more seriously and that's the good news.
It's just difficult to arrest the scammers and even harder to prosecute them. When law enforcement can do more because they have stronger laws to work with, then more arrests can be made. In the meantime, we have to settle for what arrests we can get. And we do try very hard to get them arrested.
Silence is the scammer's best friend; knowledge is the scammer's worst enemy. 沈黙は詐欺師のよき友達、知識は詐欺師の天敵。Think globally, act globally.