The reason an election doesn't simply cause "the winning" party to take power is because, outside of two-party states, we need to give the player a chance to build a ruling coalition. Say an election split the vote 50/30/20 between the Conservative, Free Trade, and Religious parties. I might not want the Conservatives in charge at all, and decide the Free Trade + Religious parties result in sufficient Legitimacy to get by - or I could sideline the Free Trade party entirely by putting the Conservative and Religious parties in charge. This is hardly even "gamey", these kinds of post-election negotiations that set the terms for what might be politically feasible during the upcoming mandate period are virtually the norm in almost all democratic countries. We also permit free Interest Groups unaligned with any party to support a ruling party, if for example the Armed Forces have decided to stand outside party politics entirely but still have considerable Wealth-derived Political Strength due to consisting mostly of Aristocrats and well-paid Officers and supported by high-ranking Generals. So player input into which coalition should form (and support) the government is necessary.
Normally, when kicking an Interest Group (or a Party, along with all its Interest Groups) out of government, it gains a bunch of Radicals who are displeased with being removed from power. Just after an election, this penalty is revoked for a single reformation of the player's government, incentivizing using this opportunity to optimize Legitimacy in light of these new election results.
On the subject of the fine details of electoral systems (length of mandate periods, proportional vs first-past-the-post, upper and lower houses, etc.) these details are very interesting and we've played with laws that govern some of this in the past. The problem was that they were hard to balance so they mattered as much as the other laws, and they did not feel impactful to enact. Not implying here that the details of electoral systems are unimportant - I've certainly had my share of lengthy debates of the impact of MMP vs FPTP - but in the simulation the impact isn't felt as tangibly and doesn't affect gameplay as much as, say, extending the voting franchise to more Pops or instituting a welfare program. So to do this justice we'd probably need a system of sub-laws or configurable laws, which isn't something we'll be able to do for release at least.
