They spent this huge amount of money on their team and Bayer Munich is definitely the richest team in Bundesliga and maybe one of the richest teams in the world that's why a team like Dortmund or Leipzig can't stand against them.
If you invest more you can expect to see more wards and that's what we saw in Dortmund and Bayern Munich.
Just check the last Bundesliga champions and you can see Bayern is the champion of Germany for years.
Dortmund just get closer to Bayern Munich before dropping at some point and letting them advance alone. This season was an epic one since Dortmund gave literally the title to Bayern Munich at the very end after being in advance..
The other seasons weren't different with sometimes Bayern being dominating and sometimes close to the others. Even Leipzig was fluctuating badly. Schalke were runners-up many times in the past and now they are nowhere in 1.Bundesliga..
in my understanding between top teams like Bayern Munich, Dortmund and Leipzig these three teams have always been closer in the last 3 years but Bayern Munich has always topped the standings in this bundesliga even though it is clear that several seasons Dortmund has also been closer to Bayern Munich and Leipzig have also come close to Munich's points but all of them have failed to top the bundesliga standings.
I just thought a little if it was only because of the wealth owned by Bayern Munich that it was able to provide the best position, but why in several matches it seemed clear that it seemed as if other teams had hopes like Dortmund.