The obvious answer is because it is not safe anymore in Syria. The second reason, because of the war most Syrians are not able to provide for their families anymore.
In the picture: My 10 years old Syrian cousin once removed plays with her kite somewhere in Germany. She traveled with her mother from Damascus to Germany crossing Turkey, Greece, Bulgaria and Austria. She endured a terrible journey similar to the ones you see and read about on the media.
Here is another reason many people are not aware of, because of war, Syrian men in their twenties have to join the Syrian Armed Forces. Refusing to fight for the regime is punishable by execution. Others chose to fight against the regime by joining other groups. Some of these young men wanted out for different reasons. They maybe got tired, or were injured, or even forced to do things against their well.
Sadly, some people including some Syrians who live in abundance outside Syria do not find the aforementioned reasons enough to leave Syria. Their excuse is these boats are too dangerous. Yes, they are. However, they have to make this decision not us.
All my cousins-except three- and my cousins once removed and an uncle lost their homes in Syria. Before leaving Syria, they either moved to their parents’ houses or rented apartments in quasi-safer areas. There are four millions internally displaced and even those who left Syria were as well displaced in their own country before leaving. Most of my cousins are now in Germany, France, Libya, Egypt, UAE and England far away from their beloved ones. In addition, my sister who used to live in Syria before the war had to move to Jordan.
Unless the world puts an end to the Syrian war, the refugees will keep coming. Building walls will deter some but will not stop them. Unfortunately, these walls will make it more difficult on them. When you are in a war zone, you do not plan for tomorrow you plan for now and now for them is to escape war.