It's because Square Enix as a whole is not just a game developer anymore. They are a multi-entertainment business giant. SE circulates it's business not only in games, but other media as well internationally. Putting them with Capcom is like putting MLB to JLB (Japan League, for the lack of a better name). They are in the same industry, but the scale is incomparable.
Well $$$ circulation. When you have a international entertainment business, where you spew out new projects and products left and right, top to bottom, pretty much in every angle, profit becomes a different meaning. They have to use the most efficient means to produce their products. Not just about going the cheapest route, but a fast efficient route.
But isn't hiring other third party devs the same as hiring in-house devs? Not exactly. Third party devs, most of the time work on contracts. Meaning they only get paid what the contract pays out. However, internal devs must be paid out every month, even when they are not working at full potential. This means more funds are used in the longer run.
Having to spend as much to make as much, taking an efficient routes means to spend less to potentially make more. And to possibly reduce loss in case the product does not go according to plan.
The HR circulation is just on a different scale, to say in comparison to Capcom, or even CD Projekt Red, where Red is exclusively a game developer. Their profits are hard line simple.