The LAM (Landscape Auto Material) marketplace asset Developer told me, that this is a UE4 lacking feature which makes LAM lacking that auto generated trees and such things have collision. That is really a big problem and I hope Epic can follow this feature requests to make it possible to spawn trees (of course with collision!) with such auto generating functions! Of course grass normally need no collision, but when this functions are used to auto generate also trees in a good way that LAM is doing it, it should be possible with collision!