It's a bit of a market inefficiency and a product of the FM generation. 30 year old plus veterans have a ton of value and in many ways are better and more stable than younger guys with resale value.
Bayern is looking to win domestic and continental trophies, not worrying about resale value. Veterans help you win. Look at Madrid with Modric, Kroos, Alaba, Nacho, Carvajal.
That's not entirely true.
Madrid win because they have stockpiled some of the best young talent in the world over the years. And all of this has happened because they bought players with resale value and shipped them off before their value dipped.
Their experienced core have all mostly been replaced by young 20-23 year olds who are amongst the best in their position and literally all have them have stepped up over the past couple of years.
This is not a team relying on 30 year old players to win trophies. Most of their key players are under the age of 24.
Having a core group of experienced players is important. Conversely, buying promising young players (with resale value) is just as important to help you win trophies and maintain that winning consistency.