There is no (liberal) logic to first past the post

Voters tend to be opposed to the government otherwise elections would have no bearing on the state. Elections would be irrelevant if everyone is pro-state because each time the people would choose to move in the direction of more communism and a bigger state. It is because the people might oppose the government that elections are meaningful and that democracy tends to oppose communism. If democracy is not communist then more democracy would tend to weaken and reduce the influence of the state.

First past the post is less democratic than proportional representation because it offers the voter less (meaningful) choice. Because fptp is generally a two-party system voters must either waste their vote or vote tactically which means the people have much less power in a fptp election (and the government have more power). In a democracy the interests of the people (not the government) are expressed via an election. Elections are bad for the government which is why democracy is very close to anarchy and more democratic systems (such as pr) are more like anarchy (and worse for the government) than first past the post. Governments prefer for there to be very few elections and they prefer fptp to pr because it gives less power to the people.

Even in a democracy the people are not the government and so then what is good for the people (democracy) is not good for the government.