I have no idea about Beijing or Moscow, but Paris, Berlin, and London were all located on swamps/marshes.
I think the reason is simple, "Braindead" style. It's just that lizard folk prefer the marshes, so that's where we have to be.
For most of history, shipping goods over land didn't make much sense.
For some reason, capitals tend to be on undesirable rivers.
Turin-Savoy in Italy, Prussia in Germany.