It’s done by the individual states, generally for one or more of three reasons:
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to attempt to control costs and decrease the number of annoyingly self motivated (aka politically unreliable) humans involved.
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to provide opportunities for profit to favored individuals and corporations who have made significant contributions to sitting politicians.
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a fetish for modernism, a desire for cool shiny futurism.
In practice, the second factor works out quite well for those involved, the first and third, nope and nope.
My state got mechanical voting machines that punched a paper tape where you could see it, and know that your vote was properly recorded, around 1960 I think. But then around 1990 they were replaced with “modern” unauditable electronic systems (that I am somewhat intimately familiar with) and now we are tied for dead last (along with Mississippi and Louisiana) for having the least reliable voting system in the USA.
Note, we are a thoroughly Democratic Party dominated, machine politics state, and the replacement of the voting machines was done with full knowledge of their weaknesses and exploitability, in a bipartisan consensus led by Democrats.
