“But are they really that simple?”
Yes, they really are, at least the vast majority of them. And to be clear, by simple, I mean simple-minded as well as one-dimensional in their motivations.
They may do a cost-benefit analysis, but they are too stupid to include all the variables they should be accounting for. So the only real variable they seem to account for is who is going to provide more “free stuff”, and those candidates always happen to be Democrats.
That and the notion that the costs are going to borne by “somebody else.” If a welfare slug, then some probably lighter skinned taxpayer. If a government worker or contractor, then some schmuck in the real economy which has to provide real goods and services. Sure, it may mean their personal taxes go up, but they expect to get a raise or sell stuff to the government which will more than offset the tax increase.