Some of them have people here waiting to exploit them. Many Eastern Europeans had their fares paid by Gangmasters who then made them work to pay back their passage.
Entire familes pool resorces to raise the money to send a member. They are told that once in Europe the person sent will get a job that pays so much money that they can repay the 'fare' and support the rest of the family 'back home'. It is a bit like a family here saving up to send a child to university.
There is often a chain involved here - migrants will be handed on to people in their country of destination who will collect money towards the fares they have incurred.
Same applies, it is Europe that is the attraction. We end up with far fewer than Italy , France, Spain, Germany. Our newspapers and media contrate on the ones that want to come to the UK so people here think we are the main destination, but it is not true.