Corporate personhood surely does provide machines with access to benefits that they wouldn't so conveniently have if the only legal actors were humans.

Such as what, exactly? You still need at least one human, and if you control a human why do you need a company?

So: limited companies get tax breaks from the government, can sell stock and be listed on the stock exchange, and have legal responsibility which doesn't rest on any individual human. Humans are slow. Allowing automation of contracts allows for speed-up.