I was looking at an Australian site whose contributors apparently know a lot about migration and who seem to be trying to tackle some of the misinformation and widespread animosity directed towards immigrants (surprising really since all but the aboriginal Australians are themselves descended from immigrants).
There is reference to the fact that there are millions of refugees across the world and a substantial number of them are stuck in very basic and often fairly squalid refugee camps where they may have spent all their lives and where they are likely to end their lives. It was also pointed out that most refugees are hosted by poor countries. For instance, there are apparently 500,000 in Kenya, 1.7 million in Pakistan, 1 million in Iran and 400,00 in Jordan. Sadly, 1.4 million fled the turmoil in Iraq and are now stranded in the horror that is Syria.
I'm not an expert on migration and I have no immediate knowledge as to why people use the methods they do to leave their countries or indeed why they decide to leave everything they have ever known to undertake perilous journeys with no guarantee of where they will end up. I think an educated guess might be that they are desperate. Many of these people have probably never travelled far or had passports and may not even have birth certificates and other official documentation that we take for granted in this country. If you're living in one of the many areas in Syria that are being bombarded by government forces, American/UK forces, ISIS and possibly other disparate elements, unless you live near a major city which is functioning fairly normally, it is probably almost impossible to obtain these papers and a passport. I also seem to recall that penalties were introduced to prevent airlines from allowing passengers without the proper visas to travel to countries that have strict visa requirements.