Laying the groundwork for a corporate franchise while not immediately suffering from adverse economic effects would be a reason for doing it. Such as just as a major inter-continental war is drawing to a close right before the Forbidden Palace or Versailles is built.. and just before the new cities stop revolting and the newly acquired resources come into play which might justify the founding of that corporation. With some micromanagement a player could conceivably save themselves from economic ruin by delaying the effects of corporation which would have compounded the effects of the extra maintenance costs of a bunch newly acquired distant cities. Being under the economic protection of SP while the cities infrastructures are being rebuilt is very helpful. And having the opportunity to set up corporations during that time would also be helpful. The corps would have no effect yet, but once the cities are back on their feet then WHAM you switch civics and the corporation is ready for business in a big way.
Timing of Wall Street, resource acquisition through Capitulation and more.. are other reasons why a player might want to found a corp while under SP. And what about those times when a Great XXX unexpectedly spawns with >10% odds.. if you just changed civics or cannot afford to.. and if a delay might mean losing the opportunity for founding a corp... why not found it under SP if you know you will gain future benefit from it?
I don't think real life can be realistically compared here. Where would you draw that line of how State Property effects the games commerce? Its just a fun model nothing more. So realism can't take precedence over gameplay since gameplay trumps realism in this entire game. The game is modeled in reality but it is based on pure gameplay.
Sorry, but I don't consider these valid gameplay reasons. All of the above reasons are reasons why it could be
useful to build a corporation in state property, but that isn't the same as a gameplay reason to add this ability to the state property civic.
A gameplay reason is a reason why a game rule would enhance gameplay. For instance, someone might argue that it is nice if theocracy would also give the ability to build missionaries and that organised religion should stop foreign non-state religions from spreading into your cities. Both rule changes would make the civics more useful and more powerful. But in my opinion it would be bad for gameplay as the differences between these civics would be lessened and thus the game choices more shallow.
Or for instance, one could argue that it should be possible to lay down railroad tracks without having coal or oil. It wouldn't improve the speed of units until you got coal or oil, but once you got these resources, you would directly benefit from the faster movement rate. There is of course a realism argument against this change in game rules, namely that no nation would ever build railroad tracks without the ability to use them. But there is also a gameplay reason against this as it would diminish the value of having coal or oil. It would remove a part of the difference between a nation with oil or coal and a nation without these resources.
In the same way, I would see the addition of the ability to found corporations to the state property civic a way of making the game more shallow. It would reduce the effect of the choices that you make. It would matter less what civic you would be using.
If you want to give a gameplay reason, then you should give a reason why it would improve the game: more interesting choices in the game, the various different strategies which could improve the game. In the case of allowing founding corporations to every civic, I would say that it would remove a choice from the game namely the one about whether you want to use a civic that allows founding corporations or a civic which doesn't allow this. It's an important choice which you will have to make at that point in the game and it will determine various other paths to victory from that point on.