The gap between a dangerous boat crossing and a short, easy flight over the same water is often just which passport you happen to hold. Someone who travels freely could be in the boat themselves, were it not for an accident of birth.
People leave places where they are already relatively safe, because temporary safety offers no future for their children. A slim chance of a secure permanent home can be worth a deadly sea crossing and a long overland trek.
When governments close legal routes, the need to move does not go away; it is handed to criminal networks who pack boats past their limits and prey on travellers. Policies built on keeping people out are therefore both cruel and bound to fail, because they do not stop the journeys, only make them deadlier.