[Ger-Poland-Volhynia] Why Germans in Poland

[j burns]

There is quite a bit of information about why Germans ended up in Russia-Catherine II and Alexander I invited them, the partition of Poland threatened their military exemptions, Polish insurrection, abolishment of serfdom, etc. but I can find any information on why there were so many Germans in Poland in the first place. Can someone clue me in please?
John