That's because of the local rivalry. The same with us and Espanyol, while you might see some cantera players from Espanyol joining us at a young age, you rarely see adult Espanyol players joining us and vice versa (the quality of Espanyol players might not be top notch but still they wouldn't sell them to us).
I don't think it's in any way related to "wanting to sell". Teams in Germany don't "want to sell" to Bayern, Bayern simply knows opportunistically when players contract run on 1 year down, or when they have a cheap release clause. If Barca had any plans to loot Espanyol over the next 10 years, they'd absolutely ruin them and theres little Espanyol could do.
If you look at any team or league, the chance that players stay there and sign long term contracts is proportional to the amount of money that team has at disposal.
So really there is only single factor we need to look at: How much budget do these teams have.
Here in Germany 1 team has a big budget. The others fall flat.