In many cases, because both overlap with geographical features: the Carpathians in Romania, the Pontic-Caspian steppe in Ukraine, the green mountainous Portugal north of the Tagus vs the flat "montado" landscape to the south.
>because both overlap It's easy to see why old empires/countries were divided by geographical features due to military reasons. But why would it affect the current-day political climate directly ? I think the causal relation is more like Geo -> old empire -> (more things) -> current day politics like the original post implied, than Geo -> old empire Geo -> current day politics as you implied; unless I missed your poi…
geography → economy → old empire
geography → economy → current day politics
The biggest caveat to this is that the importance of geography is conditioned by technology, and technology changes over time. So a more detailed model could be something like: geography + current tech → agriculture/commerce/industry/military → economy/culture → politics
See how the dominant powers changed with the shift from bronze to iron or with the development of open ocean navigation.