Hard disagree here. Munich, Berlin and Hamburg simply don't have much land in their borders any more. In Munich, we only have two major unpopulated areas left - the SEM Nord (900 ha, 50.000 people) and SEM Nordost (600 ha, 30.000 people) [1], and the currently under construction projects Freiham (25.000 people) and Bayernkaserne (15.000 people).
The opposition to these projects does have valid points, too: a city as large as Munich absolutely needs recreational areas for the populace as well as green areas for micro-climate and recreation, and the amount of both is really really low - the traffic to the existing lakes or the nearby Bavarian Alps is already immense and the areas are all struggling with the regional-tourist population issues like overcrowded trains, trash being left everywhere including in natural reserves or on farm land [2], overcrowded parking lots and vandalism [3], leading to sometimes outright violence against tourists from Munich [4].
"Just build more dense housing" may sound like an easy way out, but (too) dense housing has immense followup costs as well as all these people have to have other options in their lives than slave away as corporate drones and sleep. Otherwise, you end up with Japan-style psychological issues.
[1] https://de.wikipedia.org/wiki/St%C3%A4dtebauliche_Entwicklun...
[2] https://www.abendzeitung-muenchen.de/muenchen/umland/ausflue...
[3] https://www.merkur.de/lokales/region-tegernsee/dasgelbeblatt...
[4] https://www.abendzeitung-muenchen.de/bayern/tourismus-in-bay...