Because of mass protests.
Because lawmakers have access to the best healthcare, wear their masks for the TV cameras, and then do the opposite when no one is watching.
Because politics is a social game of meetings, meals, drinks, and parties.
Probably many more reasons. Those are just a start.
You said effectively, I don't think that's true because given "wearing a mask": "protection of others" > "protection of self".
This point and also the non-sequitur "sometimes people are selfish" can both be true, and yet the original point "not wearing masks" => "more infections" can also still be true. Just because masks protect others more, doesn't mean they protect the wearer zero or negative. Just because peoples' motivations are suspect doesn't change the arrow of causation.