A lot more precision and control goes into those cake mixes than the combination of ingredients you are likely to use at home. For baking in particular this matters if you want consistent results. The ingredients themselves are not all that special.

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mattkrause 2 days ago | parent | next [-]

For cakes, you often want less gluten (~7-10% protein) and that flour is not super common. All-purpose flour “works” but the product will be a little bit tougher because it a bit more protein instead.

	▲	bigstrat2003 2 days ago \| parent \| next [-]
		That may have been true at one time, or it might depend on location. But at least in my area, cake flour is super common. Every grocery store has it. It's not generally worth the hassle of stocking it in my pantry versus just using AP, but when I'm trying to go all out I can get cake flour no problem.
	▲	massysett 2 days ago \| parent \| prev [-]
		Cake flour, any full-line supermarket will have it.

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ksenzee 2 days ago | parent | prev [-]

As others have pointed out, you do need low-gluten flour, which you can indeed buy at any US grocery store, but it’ll cost more than just buying cake mix, unless you make a lot more cakes than anyone I know.