I'm dubious of the idea that I should be training mentally in an area that a computer program can already trounce all humans in. Playing optimal poker is computationally solvable and not demanding, except for figuring out the biases of the human players.

You obviously have very little knowledge of the topic you're presenting yourself as an expert on.

Do professional poker players make most of their money playing against rubes, or against other professionals?

As is a logical necessity, most make their money off of non-professionals. The very best probably make more off of other professionals, though there are some very bad players even at the highest stakes (e.g. billionaires who just like poker).

Is any of the money counted as their earnings prize money contributed by sponsors, or advertising sponsorships?

There isn't good public data on sponsorship deals. When one hears how much a particular professional makes, they are seldom included.

What are the arguments that a human can outplay a computer at poker?

The fact that you don't see computers beating the best humans (except in some somewhat marginal forms of poker, and even there it's debatable), and in most forms of poker, not even the semi-good players.

This isn't a matter of "argument", but a matter of observing the facts.

How is poker more useful than practicing multiplying large numbers together?

You don't usually get money for simple multiplication.