It's pretty obvious though isn't it. Bayern have been utterly dominant in German football for years. When that happens, others look on for who will be the team to challenge them.
When Dortmund got close, their 3 best players joined Bayern. Now, their main challenger is Leipzig who have average players but one of the most sought after managers in Europe and Bayern just take their manager.
At the height of the United/Arsenal rivalry, if Fergie left United, would you have expected Wenger to take his place, or Pep to join Real after he left Barca?
For a German player or manager who wants to spend his best years in Germany and play/manage at the highest level, there is one option. That makes it an uncompetitive league.
As an outsider looking in, nobody cares who Dortmund sign from other German clubs because they haven't won the league 9 years in a row.
That's a different topic, no one is arguing that Bayern aren't head and shoulders above the rest of league in terms of prestige and money or that the league is exceptionally competitive. I responded to two claim that were made, first that the Buli was happy to help Bayern dominate and that Bayern were always poaching players from their rivals both are wrong. Bayern aren't rivals with the other teams mentioned not like Real Madrid and Barcelona are so the comparison is misguided particularly when Barcelona and Real are in the same bracket in almost all criteria, Bayern also don't really operate within Bundesliga for big players, that's why people essentially mention the same three players and it should be repeated, Hummels came from Bayern, Gotze come from Bavaria and by signing Lewandowski they prevented him from strengthening CL competitors because Lewandowski was leaving Dortmund no matter what.
Regarding United, at the height of the rivalry United and Arsenal were similar clubs, Leipzig and Bayern aren't rivals, they are not equal to Bayern and it's a step up for Nagelsmann. When Arsenal lost ground, their best player left for United. United are also a poor example, we are talking about the club that used to handpick the best players in the PL from Keane, Cantona, Cole, Yorke, Ferdinand, Berbatov, Carrick, Rooney to prime Van Persie, United actually operated heavily within the PL. That's exactly what happens when you have money and prestige, the best local players choose you over other clubs.
Bayern dominates because they are more competent and more stable than everyone else, they don't really poach other german clubs, other german clubs and fans don't bend over for Bayern, they are simply ragdolled by Bayern and there is nothing they can do about it outside of trying to improve organically.