Because I find the idea of corporate superpowers (Especially the way done in Shadowrun) to be dumb. Megacorps are to real corporations as Sauron is to real-world medieval tyrants. While I like Sauron, he exists more as a metaphorical embodiment of evil propelled by magic than a realistic character. Shadowrun's corporations are written to be more intimidating for the average reader and embody evil rather than have any sort of realistic grounding or logic.
It's like when people say 'WWII or FDR ended the great depression' without asking how or why. It's repeated so much it's just assumed.
So Corporations today don't have their own Communication satellites, their own intelligence operatives, their own security forces, their own Banks? Of course they do.
You fail to understand, I think, what extraterritoriality means. It doesn't mean that Corps get a large chunk of the geography and have to run it like a government. It means they get certain areas, like embassies, where the laws of the resident country do not apply. They're not burdened with any social programs, their entire "citizenry" are productive members of the corporate society, or they find themselves fired. Many corporate citizens aren't even in extraterritorial housing, so the host government has to support them with police, fire, EMT, garbage, utilities, etc. The Corps get the best of both worlds. They get a workforce that no government laws can protect and the government in most cases have to still pick up the tab.
It's no different than getting cities to pick up the tab for stadiums or counties and states to pick up the tab for Amazon Distribution Centers, or at least pay for the privilege with tax breaks. It's just a larger scale, because governments are weaker, and corporations are richer.