Dortmund is the second most popular German club. Let's equate their status to the big six in England - supporters expect Champions league every year and competing for top of the table most years. Dortmund can accomplish that in Germany and still play a lot of youth because Germany is more top heavy than England. Germany has 50+1 and the EPL just generates a mountain of additional revenue.
Dortmund can sprinkle 17-19 year olds into a group of veterans, be somewhat inconsistent and still finish second in the table. They can generate a high profile for youth talent which can yield a bigger payday down the road for the player. The downside is that the players still end up leaving.