"Any local connections" is a bit of a distortion. In any case, 13 local players in one county squad really isn't bad.
And for what it's worth the imports tend to be long-termers - eg Adams, Goodwin, Mushtaq.
Birthplace is, to a large extent, an irrelevant accident. You say that "most people are born and schooled in the same place" but the example of Sussex is, I imagine, pretty representative: for every 9 locals born in the county, there'll be about 4 who weren't.
In my family, my brothers and sisters and I were born in a total of (I think) 4 different counties. Fortunately, I was born in Sussex!
It may also be that the population of a places like Sussex is boosted by an influx of older people. I imagine that more people die in Sussex than are born there. A fair number of people live in cities when in their childbearing years, before moving out to the country in later life. That's certainly my plan in life, as a result of which my daughter was born in London.