Because the political power isn't a problem in itself, in my opinion, only because of the tie to faction unity which is entirely arbitrary.
Every mechanic is arbitrary.
As the OP framed it, making the foundation upon which faction unity is generated stem from population size (of faction pops) rather than being inflated via political power also seems more balanced.
How? I'm begging you, repeatedly,
tell me why you feel this way. You keep saying, this, but you just... assert it. And then cite SB/UA as evidence. But SB/UA is easily addressed through other means.
Why is more balanced to base faction unity on pure pop counts?
Pops are already part of the equation, but the size of a faction becomes irrelevant when its impact on faction unity is warped by political power.
... Yes. Are you basically saying "why does this thing that's meant to represent internal
politics care about
political power?" The point of faction approval is to align your (the player's) incentives with roleplay. Your egalitarian faction will be happy if you have egalitarian policies and do egalitarian things, and when you offend your citizen's egalitarian sensibilities by e.g. allowing population controls, you are punished with missing unity. The fact that the only people you care about are the people whose opinions matter (the one with political power) is a triumph of storytelling through game mechanics, not an issue (in my opinion).
Right now you have certain living standards providing more impact alone on faction unity than e.g. Parliamentary System combined with UA or SB does, so grabbing a civic like that is like a drop in a bucket in comparison, limiting meaningful niches and choices.
Again, you keep coming back to UA/SB. As was proposed, literally as the first response in this thread: it's so much simpler to just increase the political power of SB/UA. Give UA +300% political power, and UA with the Shared Burdens Civic will be the best political power in the game, all the time.
It's worth noting that this change, where you make factions only depend on pops, would affect every living standard
except UA/SB. Whatever numbers you decide on for UA/SB, you could adjust their political power in the current system to have exactly the same behavior. Literally exactly the same, since UA/SB already are at this state, where faction unity depends purely on pop count. This change would
only affect the living standards that
aren't already equal in political power per pop. It would have no effect on SB/UA gameplay.
The solution to realizing this, in my opinion, is to sever that tie and level the playing field... from where you can then more easily balance e.g. the living standards on their other merits without having to consider a hodge podge of political power values of each living standard. It makes designing and maintaining this system in the future (internal politics rework when?) a lot easier, this being the real loser of overly complex mechanics.
Ok, so the issue
is that it's too complex for what it gives? But you explicitly said that wasn't the issue before.
Exactly. The opportunity cost for running SB is so much higher, and because of this it currently makes a lot of sense to remove the civic too alongside ditching the living standard later on. But, with the change I've been describing this would also provide incentive to keep the civic around, reaping the rewards from added Progressive faction unity gain as it then would actually be somewhat significant compared to the current state of the faction unity meta. This in turn would make boosting governing ethics attraction, a normally insignificant mechanic, provide some synergy in maximizing the benefit of egalitarian societies.
Again, the SB thing:
boosting the political power of all pops under SB/UA would fix this. It would give you a fairly normal amount of faction unity for the CG you pay before accounting for the civic, and the double egalitarian unity would be a bonus on par, or better than, Parliamentary Systems (which stacks with, multiplicatively, if you want to do both). It would make SB a more flavorful version of Parliamentary Systems, that you can upgrade later in the game to be even stronger.
And governing ethics attraction already
is a significant mechanic, in all societies. The more pops are in your happiest factions instead of random fringe factions, the happier all your pops are (giving you more stable planets) and the more faction unity you make, even if that amount of faction unity is low. If the problem is that it's not strong enough in SB because SB faction unity is too weak: buff SB.
A much simpler to understand, simpler in practical effect, less gamey and arbitrary of a system.
<repeat paragraph about how political power is exactly the right mechanic to build your system representing internal politics around, how it makes sense in-universe, and how it makes your incentives align with roleplay>
It would certainly be simpler. Removing things makes the game simpler. But if you remove everything, you have no game left. It's not just a question of whether it's simpler, it's a question of whether it's
better. Whether the complexity a system adds is not worth the value it gives.
And every system is arbitrary, from the specifics of the faction system to the colors of the icons.
What you're proposing is 1 pop-1 vote, regardless of the political structure of your society (Auth, Materialist, Xenophobe where no one but the main species are citizens, etc.) would make more sense. It's already 1 pop-1 vote for UA and SB. So
sell us on why it should also be that way for Stratified Economy and Academic Privilege (which is what you're proposing, by untying the two mechanics).