The reason is simple. Swamps are commercially strategic, since they're located on rivers.

For most of history, shipping goods over land didn't make much sense.

This makes perfect sense, I guess they are swamps because that's the combination of rivers and flat ground

We would have Berlin on Rhine and Moscow on Volga river then. Much better climate and much busier rivers commercially.

For some reason, capitals tend to be on undesirable rivers.

...if this was the only condition for "what city becomes a capital." Looking at Germany, there's Hamburg, Köln, München...all on nice rivers, but none of them were the capital of Prussia when Germany was first unified - since Prussia was the essential driver of that political process, Berlin became the capital. Something of a coincidence in the grand scheme of things, really.

For some reason, it's usually a faraway landlocked province that triggers unification. Not the rich and coastal ones.

Turin-Savoy in Italy, Prussia in Germany.

Hence the "Drain the swamp" political rhetoric of the last US election cycle.

FWIW, Berlin and Moscow both mean "Swamp Town".

Canberra also.