> What's the argument here?

The most common argument for this is some core unexamined axiom that only humans can reason by definition, and then working backwards to a justification for that.

Not only unexamined, but ineffable. I've yet to see anyone give a good definition of what they think "reasoning" is that applies to humans solving complex logical problems but not to machines.

(I suppose a religious or otherwise superstitious person might introduce the soul into this, but I haven't come across anyone actually willing to defend that hill.)