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..