The head of HR explained to me once why they can't recruit locally by saying "nobody grows up dreaming of working in a chicken factory ".
And Polish and Latvian kids do? 
Dreams and vocations aside, most of us end up doing whatever pays the bills the best, within our capabilities and limitations.
Working in places like abattoirs is never nice but if the money is right they will come. It’s all relative.
Where I used to live there were people from India and Nepal who had left their families behind to stand in burning hot car parks all day hoping to wash shoppers’ cars at 3 quid a pop, a large chunk of which would go to their company sponsor. I doubt they did more than four or five each per shift as there were always far too many of them working a small area. Or working as toilet attendants in hotels, or cleaners etc. They probably made in a month what dH and I could spend on a good weekend out. It was worth it to them compared to the lack of opportunity or decent pay at home. Whereas the local population quite literally would not get out of bed for less than 100k a year, because they are so well looked after by their rich government who only spend money on looking after their own, that they would see it as demeaning to have anything other than a well paid job for life, with retirement at 50 on final salary pension.
It may be an extreme example but it really puts into perspective the disparity of earnings from country to country. I’ll never forget reading on a Facebook page for the city I lived in, an Indian chap asking how much he needed to earn to have an okay life there and still have plenty to send home to his family. . Another Indian replied ‘to have the equivalent of middle to upper middle class lifestyle at home you’d need to earn XXX a month here.’
That XXX was about 450 quid. Now admittedly his idea of upper middle class may not be ours, but even so...
Who used to process the chickens in the UK before the Eastern Europeans came? Who used to pick the crops? Who used to mop floors and push the hospital trolleys? Wait tables in chain restaurants as coffee shops? How come we suddenly don’t HAVE enough people who are willing or able to do these things? Where did they all go? 