Airlines already exist ingame as "Air Transport Corporations."
They operate in much the same manner as any other corporation--the primary difference being that they utilize civilian airports, not production plants like other corps. To add what you suggest would require that each corp type in the game have that sort of micromanagement, unique from other types of corps.
For example, should "services" corps be subdivided into insurance, fast food, and hair salon chain corps? Should you have to decide what types of policies to offer on an insurance services corp, and set prices (premiums) based on actuarial tables?
Should construction corps do residential or commercial construction? Should they have their own mortgage departments? Should they engage in land speculation?
Should oil corps have to constantly balance new exploration with current oilfields running out? Should they have their drilling platforms damaged by such things as hurricanes? Should they have to micromanage their distribution network, and build pipelines?
Your idea opens up a Pandora's box of micromanagement permutations for every corp type imaginable. While Simcountry is certainly an in-depth game, I doubt many players would have this kind of time to devote to EACH of twenty, fifty, two hundred, five hundred corps within their empire...to say nothing about CEOs! It seems good in theory, but in practice, no one would ever be able to step away from Simcountry lest all their corps crash from lack of attention.