Given that most taxation done with the the advertised goal of helping the poor in Australia does happen with popular assent of individuals, I would theorize your position is false and that people do have some individual incentives to develop services offered to the poor -- for profit, humanitarian / charitable reasons, and a variety of other considerations.
I never said that wasn't the case but historically that "some" was never sufficient. On average people are rational and selfish to a larger extent than they are altruistic.
> taxes in that pursuit are illegitimate under a theory taxation happens by the assent of the people
Well the whole concept of an organized society falls apart if individuals can personally freely chose which laws to obey and which taxes to pay. You have to have a balance based on a reasonable consensus, otherwise you end up with totalitarianism or anarchy (and in that case the people who have the means and resources to do that will establish alternative power structures and will end up subjugating those who do not AND also outcompete those how have the means but are unwilling to do the same).
It can't simultaneously be true that most don't want to help the poor at their own cost while also the tax has been legitimized by the majority wanting to help the poor at their own cost. Only individuals or representatives elected by individuals can vote and if their incentive was to not help the poor then they'd vote not to and that would be that.